Question: what is a galaxy

Paul Higgins answered on 14 Nov 2012:

A galaxy is a cluster of stars, dust, gas, and other celestial objects (black holes, supernovae, neutron stars…) that is bound together by gravity. Most galaxies have a supermassive black hole in the center. There are different types: elliptical, spiral, irregular… We live in a solar system, and the solar system is one of many in the galaxy we live in: the “milky way”. The mystery about galaxies is that stars near the outside orbit at nearly the same speed as those closer in (they should be going a lot slower). A lot of people think that this has to do with dark matter, or that we do not fully understand Einstein’s laws of general relativity.