Astrophysicists have just released the most advanced data yet on the early Universe - in essence they have produced a baby picture of the Universe. The results are from the European Space Agency's PLANCK satellite, and measured microwave photons that were produced over 13 billion years ago, when the Universe was only 370,000 years old.

Although background radiation such as this appears to be just noise and static to most electronics, it actually contains quite a lot of information on the early Universe. By recording the energy distribution of photons in each direction in the sky (or in other terms, measuring the temperature of the radiation) physicists can determine the properties of the cosmos shortly after the Big Bang. Regions of colder microwaves indicate clumps of matter which would eventually form into galaxies, and depending on the shape of the clumps we can tell if it is normal atomic matter or the still mysterious dark matter.  By measuring the angles between the directions of regions of similar temperatures, we can determine the overall shape of the Universe and how smooth it was initially. 

And the new data from PLANCK has strengthened the evidence for the Big Bang model and the parameters measured over the last two decades by similar astrophysics probes. It has slightly altered the age of the Universe from 13.7 billion years to 13.8 billions years. It has also confirmed that of all the energy in the Universe, normal atomic matter (which is the only type we have any experience with on Earth) accounts for only about 5%. Roughly 25% more is a form of matter which is affected by gravity but does not have electromagnetic interactions - this is the mysterious dark matter. The remainder is an even more mysterious substance called dark energy, and which is causing the expansion of the Universe to accelerate. (NB: The published percentages are actually more precise than I have given here, but there are a few exotic models in which the numbers are slightly different)

So in summary, the state-of-the-art cosmology as of today is that something happened 13.8 billion years ago that produced a Universe smaller than a single atom. There are many competing theories, and experimental physics and astronomy is not yet at a point where it can describe the actual Big Bang itself. For a tiny fraction of a nanosecond, the Universe expanded slowly and was at temperatures beyond any in existence today. Then tiny particles called inflatons appeared, and they caused the Universe to inflate by many orders of magnitude in under a second. The inflatons decayed into other particles, and reheated the Universe. As it continued to expand, the temperature dropped and this cooling caused all of the known species of particles to precipitate out of the primordial fireball. When the Universe was a few minutes old, the protons and neutrons formed and entered into high temperature nuclear fusion reactions, forming the nuclei of the lighter elements (such as deuterium, helium, and lithium). As the Universe cooled further, these nuclei caught electrons to form complete atoms. A million years later or so, gravitational forces have pulled the matter into clumps that will form nebulae and galaxies, which in turn will form stars, whose nuclear fusion reactions will develop the heavier nuclei in the Universe. After several generations of stars have formed, burned brightly, and then exploded into supernovae, there was enough matter to form planets such as ours, where a few billion years ago life formed. A lot of history happens, until astrophysicists send up a probe to catch light from the early Universe that is just reaching us now.
What a fascinating Universe this is.