WHAT IS A GALAXY?
A galaxy is a huge gravitationally bound system containing stars, stellar remnants, gas, dust and dark matter. Galaxies range from relatively small dwarf galaxies to enormous systems containing hundreds of billions of stars. They can also contain supermassive black holes at their centers.
HOW DO GALAXIES FORM?
Galaxies began forming in the early Universe as matter gradually collected under gravity. Small structures merged together over billions of years, growing into larger galaxies. Dark matter played a major role by providing gravitational structures that helped ordinary matter gather and form stars and galaxies.
THE MILKY WAY
The Milky Way is the galaxy that contains our Solar System. It is a barred spiral galaxy with a central region surrounded by a large disk of stars, gas and dust. The Solar System lies in one of its spiral-arm regions, roughly 26,000 light-years from the galactic center.
GALAXY COLLISIONS
Galaxies can interact and collide because of their mutual gravity. Despite the enormous number of stars inside galaxies, direct star-to-star collisions are extremely rare because the distances between stars are so large. The Milky Way and Andromeda are expected to interact in the distant future.