A galaxy is a large grouping of billions of stars, planets, and dark matter that is held together by gravity. Galaxies form into three general shapes: spiral, elliptical, or irregular. Spiral galaxies contain a significant amount of gas and actively produce stars. The majority of galaxies are spiral galaxies. Elliptical galaxies, which can be circular or elongated, are often older galaxies with less gas, producing few to no stars. Irregular galaxies seemed to be most common in the early creation of the universe.
Galaxies often exist in clusters. When two galaxies get too close to each other, their gravitational forces cause them to eventually combine. When this happens, existing stars generally do not collide with each other, but the gases combine to encourage the birth of many new stars.
The Milky Way galaxy (our home) and Andromeda are on a path to collide in 4 billion years.
