In smashing two contra-rotating beams of gold nuclei into each other scientists have managed to create the conditions which existed 10 micro-seconds after the Big Bang explosion. The direct collision of two gold nuclei at an energy of 100 GeV produces a fireball of 2×1012 degrees Celsius, 300 million times hotter than the Suns surface. Within the fireball over 1000 quarks are unleashed. When by small chance two of these quarks hit head-on, their kinetic energy is turned into matter allowing a virtual particle/anti-particle drawn from the vacuum to attain actuality, whereupon the particle/anti-particle pair fly apart at right-angles to each other, and as they do so, tear more particle/anti-particle pairs from the vacuum in a cascade, producing two jets containing many particles. These jets are detected and measured in the surrounding apparatus. But the jets, which were expected to emerge un-scathed from the seething cauldron, are impeded by the quark-gluon plasma, which acts like treacle slowing the emerging jets of particles down asymmetrically, the jet closer to the edge of the quark-gluon plasma being retarded the least. The quark-gluon plasma lasts a brief 10-23 seconds, but long enough to severely restrict the nascent jets. This hindering was entirely unexpected. Previous calculations had suggested that the vacuum structure would melt easily as the energy exceeded 170MeV, the energy at which the quark-gluon plasma should form. The quark-gluon should have been like a weakly interacting gas with quarks and gluons floating about in gay abandon barely bothering each other. The jets should have emerged with little hindrance. But the experiment proved otherwise: the jets were absorbed 10 times more strongly by the quark-gluon plasma than expected, meaning that the quark-gluon plasma is 10-15 times denser than expected, suggesting that the quarks in the plasma are interacting strongly with each other and with the surrounding gluons and moving more coherently with each other which is more indicative of a liquid than a gas where the particles move randomly. In fact, this liquid is the most ideal liquid ever observed, being 10 - 20 times more liquid like than is water. It seems that bound quark states, such as charmonium, a meson which consists of a charm quark and an anti-charm quark, can still exist in quark-gluon plasmas at energies twice as great as 170MeV, where the vacuum was supposed to melt. This means that the vacuum is more resilient than thought, not melting so easily, but still guiding the quarks in confined paths binding them in pairs (mesons) or triplets (baryons). Moreover, this high-temperature vacuum plus quark-gluon plasma seems to bind quarks in new combinations creating new particles that would not be stable in our everyday world.
As the fireball expands and cools, this vacuum structure exhibits a phase change, reverting from the less-ordered state of the plasma to a more intricate and ordered vacuum structure of the everyday world. At this juncture, the exotic quark pairings and combinations re-align themselves into the more familiar highly stable triplets of our everyday world such as neutrons and protons; like amorphous water turning to ordered ice crystals, releasing latent heat in so doing. The freezing of quarks into highly bound structures. This is what happened in the first 10 micro-seconds of the Big Bang, the quark matter turned to baryonic matter. [But it was still far too hot for those baryons to coalesce into bound nuclei. That would have to wait until the Universe had expanded and thus cooled some more]. In those first 10 micro-seconds, when the vacuum was molten, the Cosmological Constant was perhaps 1080 times greater than what it is today. Then 10 microseconds later when the vacuum froze, the Cosmological Constant plummetted by some 80 orders of magnitude to the value observable today; this was probably a consequence of the latent heat (the energy required to melt the vacuum) being released. This transition from quark-gluonic plasma to baryonic matter may well have some connection with the dark energy of the Universe, but see Casimir Experiment.
It was then discovered that the whole Universe was not static after all but expanding, with distant points receding at a velocity in some proportion to the distance from any observer. This can be understood if it is the space itself that is expanding. If each region of space expanded at the same rate, this would push further regions of space (which are themselves expanding) away at a velocity proportional to their distance from the observer. Thus nearby points might be receding at only 1m/s whereas the most distant parts of the observable Universe were receding at the speed of light. Those points beyond the observable Universe were receding faster than light-speed, and we could never ever see them. It must be remembered that it is the space itself which is expanding, light always travels at light-speed through space, and it is the expanding space which drags the light with it. The light from any point in the Universe is thus red-shifted (the wavelength is increased and frequency reduced), with the red-shift of the object being in some proportion to its distance from any observer. Those points at the edge of the observable Universe are red-shifted to zero frequency, where light is 'black'.
It is also obvious, that if distant regions are receding at a velocity proportional to their distance from an observer, then distant regions are less dense with matter than nearer regions. Although matter is energy (E=mc2), matter may not be the only form of energy in the Universe, for instance, space may be negative energy - and negative energy repels. So if the Universe contains equal amounts of positive energy and negative energy, the net result is that space will continue expanding, but at an ever slower rate, such that after an infinite time the Universe will just have stopped expanding. Such a balanced Universe would be flat, it's space Euclidean. If positive energy dominated, then the extra attractive force provided by this energy would curve space spherically, making it closed, and in some finite time stop expanding. If negative energy dominated, then space would expand at an ever increasing rate, giving the Universe an open geometry, where space has hyperbolic curvature, and the Universe would expand indefinitely at an exponentially increasing rate.
Astronomical observations showed that although the Universe was expanding, it was also very nearly flat; space was nearly Euclidean, with matter seemingly, on average, very evenly distributed throughout. The uniformity of the microwave background radiation, which is the (highly red-shifted) remnants of the Big Bang explosion, bears this out. The uniformity of this radiation was astonishing, when huge variations across the sky were expected, given that any slight quantum fluctuation in matter density at the Big Bang would self-seed and grow into enormous variations during subsequent expansion.
Another new theory then came to the rescue. Inflation. According to the inflation theory, just after its inception at the Big Bang, the Universe underwent a burst of hyper-expansion, at a phenomenally fast rate such that each point grew away from nearby points such as to be physically separate regions of the Universe, disconnected. This had the effect of ironing out any slight density variations such that afterwards the Universe was extraordinarily flat, and space Euclidean. This inflationary period did not last long, and thereafter the expansion of the Universe continued at the very much more sedate pace that we see today. Since the geometric effect on the expansion is now negligibly small, it is the matter alone that accounts for the current expansion rate. By measuring the expansion rate (the Hubble constant) the average density of matter in the Universe today is found to be approx. 10-29grams/cm3. This value is known as the critical density. Above that density, the Universe space would be spherically curved inwards, and the Universe closed. Below that value, space would be hyperbolic and the Universe would be open. At the critical density, space would be flat, Euclidean, and the Universe expand at an ever decelerating rate.
However, recent measurements on the density of matter in the Universe have revealed that it is only about 4% of the critical density. The Universe should be grossly hyperbolic and flying apart, but observations suggest that it isn't! However, only baryonic matter can be detected. If the Universe is nearly flat, then 96% of the mass and or energy must be invisible to us, un-detectable, non-baryonic. Such invisible stuff is called 'dark matter', and 'dark energy'. The dark matter will attract, like gravity, but dark energy could under certain circumstances repel, like anti-gravity. Observations on the orbits of stars within galaxies and on the gyration of galaxies in galactic clusters suggest that there is much more matter in or around a galaxy that what can be detected directly. This must be partly dark matter, plus some ordinary matter locked away in Black Holes, known to inhabit the centres of galaxies, plus neutrinos now that they have been proved to be not mass-less.
The WMAP data now suggests that the Universe is composed of 4.6% baryonic matter, 22.7% dark matter, and 72.8% dark energy and is hyperbolic, forever expanding. 6 billion years ago the dark energy contributed an accelerated expansion of the universe. The dark energy appears to be uniform throughout the Universe, without any gravitationally induced clumping, or anti-clumping as the case may be. This probable means it is composed of lightweight particles that are travelling fast, so as to have enough kinetic energy as to avoid gravitationally induced clumping. But it could be that dark energy is not made of particles at all, but is the vacuum energy of empty space. The Dark Energy amounts to only 1 nanoJoule per cubic metre, and phycicists have great difficulty explaining the smallness of such a very small, but non-zero, value.
The fact that empty space has some positive energy means that the vacuum of space could be un-stable, providing it has some lower energy state into which to decay. If so, then space could go prang at any moment, the change of state travelling at the speed of light until all space is in this new state. This new state could be anything. In some string theories, space has up to 6 extra highly-curled up dimensions, and these could un-furl to become infinite spacial dimensions, a space with 9 spacial dimensions. No stable structures could exist in 9 spacial dimensions, and the whole Universe, galaxies, stars, atoms, baryons, quarks might suddenly all fly apart in one massive twang. It might be that the only thing stabilizing space from such decay is the ordinary matter within that space; remove the matter and space will decay. But no one knows what form it would then take. With the Universe expanding, which is indeed removing a lot of matter from space (the matter density of the Universe) and the expansion of the Universe could bring about the decay of space, Life, the Universe, and, well, Everything... If, in one scenario, dark energy becomes stronger as the ordinary matter within space is diluted by cosmic expansion, then this larger positive dark energy (with positive dark energy being repulsive in nature) will make the expansion of the Universe accelerate, which makes the dark energy even stronger in a positive feedback loop, exponentially fuelling expansion and exponentially increasing dark energy to such a point that in 40 billion years time it overwhelms gravity and galaxies fly apart. It doesn't end there, eventually dark energy overwhelms electromagnetic energy and atoms fall apart, then it swamps strong energy and protons and quarks fall apart culminating one cataclysmic Universal explosion in 60 billion years. This scenario is called the Big Rip.
The Ghost Condensate theory can account for all three, dark energy, dark matter and inflation, all at once.