They usually have little gas and dust compared to spirals, few young stars, and no spiral structure. Irregulars are everything else. The Milky Way Galaxy is probably a barred spiral. With the exception of a few nearby galaxies, the spectra of all galaxies is redshifted, indicating that the universe as a whole is expanding. There is NO center to the expansion, and every galaxy sees other galaxies expanding away from itself according to Hubble's Law.
Galaxies tend to be found in groups and clusters .
The Milky Way and Andromeda galaxies are the two largest galaxies in the Local Group , consisting of some tens of galaxies. Rich clusters contain thousands of galaxies, and clusters are themselves clustered to form superclusters. On the largest scales probed so far, galaxy clusters appear to be organized on huge sheet-like structures around relatively empty regions called voids . Rich clusters of galaxies contain substantial amounts of very hot, X-ray emitting gas. The total mass of this gas can in some cases exceed that of the stars in the galaxies.