Recent observational
astronomy has shown that most, if not all galaxies contain a
super-massive black hole with a mass of
millions or even
billions of an ordinary
star.
Current theory suggests that these
black holes did not form later in the
evolution of the galaxies, from many stars or black holes accreting together to 'grow' the hole, but rather they were present at the
birth of the host galaxy, and are indeed responsible for it's structure.
Actually, the above might be incorrect, the collapsing cloud may be too unstable to for a huge black hole to form. Rather superclusters of stars may form, and collapse to 'medium' sized holes, which then aggregate.. I'd like somebody who knows to let me know!
After the
big bang the majority of the
universe consisted of
hydrogen, as the universe expanded and cooled, small irregularites in the overall density caused '
clouds' of hydrogen to form. Gradually these began to collapse under gravity, forming black holes of collosal size.
As the gas clouds were irregulary shaped, as they collapsed, the black holes they contained were given
spin. The black hole continues to suck in more of its surrounding hydrogen, which spirals into the hole, much like water down a plug-hole, feeding and growing as it does so. Due to the rotation of the hole, material in fact builds up in a disc around it. Friction occurs between the molecules of gas in this disc, heating the gas to millions of degrees, emitting
radiation at nearly all
wavelengths. Perhaps the
quasars that we see are merely galaxies at this period in their
evolution.
This in turn produces an effect like the
solar wind only many orders of magnitude larger, which in fact pushes the outlying hydrogen outwards. The effect is so huge, it's much like an
explosion occuring at the center of the proto-galaxy, with shock waves rippling outwards. It thought that these shock waves triggered the first bursts of
star formation.
This also provides a mechanism by which
quasars operate; the infalling material heats up so much, that it outshines the rest of the galaxy. (Also see
Blandford Znajek process for more info...)
Eventually however the infalling material is pushed so far away by the outpouring of radiation, that in no longer falls into the hole, it
orbits it. At this point the black hole ceases to grow, (i.e. suck much more material in), the central region darkens, and the rest of the galaxy settles down, to appear much as we see them today.