One of the foremost cosmological discoveries was the detection of the cosmic background radiation. The discovery of an expanding Universe by Hubble was critical to our understanding of the origin of the Universe, known as the Big Bang. However, a dynamic Universe can also be explained by the steady state theory.
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The steady state theory avoids the idea of Creation by assuming that the Universe has been expanding forever. Since this would mean that the density of the Universe would get smaller and smaller with each passing year (and surveys of galaxies out to distant volumes shows this is not the case), the steady-state theory requires that new matter be produced to keep the density constant.
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The creation of new matter would voilate the conservation of matter princple, but the amount needed would only be one atom per cubic meter per 100 years to match the expansion rate given by Hubble's constant.
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The discovery of the cosmic microwave background (CMB) confirmed the explosive nature to the origin of our Universe. For every matter particle in the Universe there are 10 billion more photons. This is the baryon number that reflects the asymmetry between matter and anti-matter in the early Universe. Looking around the Universe its obvious that there is a great deal of matter. By the same token, there are even many, many more photons from the initial annihilation of matter and anti-matter.
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Most of the photons that you see with your naked eye at night come from the centers of stars. Photons created by nuclear fusion at the cores of stars then scatter their way out from a star's center to its surface, to shine in the night sky. But these photons only make up a very small fraction of the total number of photons in the Universe. Most photons in the Universe are cosmic background radiation, invisible to the eye.
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Cosmic background photons have their origin at the matter/anti-matter annihilation era and, thus, were formed as gamma-rays. But, since then, they have found themselves scattering off particles during the radiation era. At recombination, these cosmic background photons escaped from the interaction with matter to travel freely through the Universe.

As the Universe continued to expanded over the last 15 billion years, these cosmic background photons also `expanded', meaning their wavelengths increased. The original gamma-ray energies of cosmic background photons has since cooled to microwave wavelengths. Thus, this microwave radiation that we see today is an `echo' of the Big Bang.
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The discovery of the cosmic microwave background (CMB) in the early 1960's was powerful confirmation of the Big Bang theory. Since the time of recombination, cosmic background photons have been free to travel uninhibited by interactions with matter. Thus, we expect their distribution of energy to be a perfect blackbody curve. A blackbody is the curve expected from a thermal distribution of photons, in this case from the thermalization era before recombination.
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Today, based on space-based observations because the microwave region of the spectrum is blocked by the Earth's atmosphere, we have an accurate map of the CMB's energy curve. The peak of the curve represents the mean temperature of the CMB, 2.7 degrees about absolute zero, the temperature the Universe has dropped to 15 billion years after the Big Bang.
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Where are the CMB photons at the moment? The answer is `all around you'. CMB photons fill the Universe, and this lecture hall, but their energies are so weak after 15 billion years that they are difficult to detect without very sensitive microwave antennas.

The last stage in matter production is when the Universe cools sufficiently for electrons to combine with the proton/neutron nuclei and form atoms. Constant impacts by photons knock electrons off of atoms which is called ionization. Lower temperatures mean photons with less energy and fewer collisions. Thus, atoms become stable at about 15 minutes after the Big Bang.
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Even after the annihilation of anti-matter and the formation of protons, neutrons and electrons, the Universe is still a violent and extremely active environment. The photons created by the matter/anti-matter annihilation epoch exist in vast numbers and have energies at the x-ray level.
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Radiation, in the form of photons, and matter, in the form of protons, neutrons and electron, can interact by the process of scattering. Photons bounce off of elementary particles, much like billiard balls. The energy of the photons is transfered to the matter particles. The distance a photon can travel before hitting a matter particle is called the mean free path.
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The CMB is highly isotropy, uniform to better than 1 part in 100,000. Any deviations from uniformity are measuring the fluctuations that grew by gravitational instability into galaxies and clusters of galaxies.
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Images of the CMB are a full sky image, meaning that it looks like a map of the Earth unfolded from a globe. In this case, the globe is the celestial sphere and we are looking at a flat map of the sphere.
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Maps of the CMB have to go through three stages of analysis to reveal the fluctuations associated with the early Universe. The raw image of the sky looks like the following, where red is hotter and blue is cooler:
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When we look out in the sky, we're actually looking backwards in time. Light from more distant objects take longer to reach us and thus we are observing now how they appeared in the past. We can see back a few billion years with the light of galaxies. The microwave light of the background shines from long ago in an infant universe 300,000 years old (the epoch of "last scattering") and illuminates the particle soup that existed before this time. This soup has a very smooth consistency and is composed of fundamental particles like electrons, protons, helium nuclei, neutrinos.
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As we move forward in time from the beginning of the Universe we pass through the inflation era, baryongenesis, nucleosynthesis and radiation decoupling. The culmination is the formation of the structure of matter, the distribution of galaxies in the Universe.
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During radiation era growth of structure is suppressed by the tight interaction of photons and matter. Matter was not free to response to its own gravitational force, so density enhancements from the earliest times could not grow.
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Density enhancements at the time of recombination (having their origin in quantum fluctuations that expanded to galaxy-sized objects during the inflation era) have two routes to go. They can grow or disperse.
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Structure could have formed in one of two sequences: either large structures the size of galaxy clusters formed first, then latter fragmented into galaxies, or dwarf galaxies formed first, than merged to produce larger galaxies and galaxy clusters.
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The former sequence is called the top-down scenario, and is based on the principle that radiation smoothed out the matter density fluctuations to produce large pancakes. These pancakes accrete matter after recombination and grow until they collapse and fragment into galaxies.
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The competing scenario is one where galaxies form first and merge into clusters, called the bottom-up scenario. In this scenario, the density enhancements at the time of recombination were close to the size of small galaxies today. These enhancements collapsed from self-gravity into dwarf galaxies.
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Hot Dark Matter vs. Cold Dark Matter :

Each scenario of structure formation has its own predictions for the appearance of the Universe today. Both require a particular form for dark matter, a particular type of particle that makes up the 90% of the Universe not visible to our instruments. These two forms of dark matter are called Hot and Cold.
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Galaxies in the Universe are not distributed evenly, i.e. like dots in a grid. Surveys of galaxy positions, e.g. maps of galaxies, have shown that galaxies have large scale structure in terms of clusters, filaments and voids.
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The clusters, filaments and voids reflect the initial fluctuations at recombination, plus any further evolution as predicted by HDM or CDM models. CDM and HDM models have particular predictions that can be tested by maps or redshift surveys that cover 100's of millions of light-years.
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