Cosmology
According to the standard cosmological model, the Universe started with a Big Bang about 14 billion years ago. During an early epoch of accelerated superluminal expansion, called inflation, a region of microscopic size stretched to a scale much larger than the visible Universe and our local geometry became flat. At the same time, quantum mechanical fluctuations of the vacuum generated primordial density fluctuations in the matter distribution. Gravity enhanced these inhomogeneities, seeding the formation of present-day structure.
The mass density of ordinary (baryonic) matter makes up only a fifth of the matter that led to the emergence of structure. The rest is in the form of an unknown dark matter component. Recently, the Universe entered a new phase of accelerated expansion due to the dominance of some dark vacuum energy density over the ever-lower matter density. This "dark energy" accounts for more than 70% of the mass-energy density of the Universe.
Extra Galactic Surveys
Viewed in X-rays the night sky glows brightly and uniformly in all directions. The origin of this 'cosmic X-ray background' was one of two great puzzles set by the very first observations of X-rays from beyond the Solar System by Riccardo Giacconi and his team back in 1963. (For which Giacconi won the Nobel Prize in Physics in 2002). To solve the puzzle required sensitive X-ray telescopes, culminating in the Chandra X-Ray Observatory. This X-ray glow is due to the summed emission from millions of faint quasars and active galactic nuclei that can now be seen individually in deep Chandra images.
To discover this answer required surveys of the sky away from the confusing and absorbing plane of the Milky Way, so objects in our Galaxy contribute little leading these to be called 'Extragalactic Surveys'. Even though we not know the answer, HEA (Harvard Center for Astrophysic studies) scientists continue to make new extragalactic surveys to learn more about how quasars evolve over the history of the universe, and to study even fainter sources - distant galaxies and, eventually, the first stars, galaxies and black holes to form in the universe.