Galaxies are the basic unit of cosmology. They contain stars,
gas, dust and alot of dark matter. They are the only `signposts'
from here to the edge of the Universe and contain the fossil
clues to this earlier time. The physics of galaxy formation is
complicated because it deals with the dynamics of stars
(gravitational interaction), thermodynamics of gas and energy
production of stars. For example, stars form from gas clouds,
but new stars heat these clouds, which dissipates them and stops
the formation of other stars. After recombination, density
enhancements either grew or dispersed. According to our hybrid
top-down/bottom-up scenario, an assortment of enhancements
formed of various sizes. Small, dense ones collapsed first,
large ones formed slower and fragmented as they collapsed. The
first lumps that broke free of the Universe's expansion were
mostly dark matter and some neutral hydrogen with a dash of
helium. Once this object begins to collapse under its own
gravity, it is called a protogalaxy. The firs