A galaxy is a massive complex or system of stars, remnants of stars. Galaxies are bound by gravity and contain from millions to trillions of stars. The size of some galaxies are measured in units called parsecs, others are measured in light years.
One parsec is equivalent to about 19 trillion miles (31 trillion kilometers). An average galaxy may extend from 1,000 - 100,000 parsecs. A light year is the distance that light travels in an Earth year. Light travels at nearly 300,000 km per second (180,000 miles).

Galaxies are usually separated by spaces that extend over millions of parsecs. These vast spaces are called intergalactic spaces. Despite the vast distances between them, galaxies are often organized into clusters and super-clusters.

Galaxies are usually classified according to their shape. Elliptical galaxies are round, spiral galaxies are characterized by their long, spiraling arms, and peculiar galaxies are shaped irregularly, likely because of the gravitational pull of a nearby galaxy. Scientists estimate the presence of 100 billion galaxies in the universe.

About 90% of the galaxy is made of up mysterious, invisible masses called dark matter. Dark matter is hypothetical; its actual existence is simply scientific and mathematical conjecture. Only its gravitational effects on visible objects can be observed (such as the rotational curves of spiral galaxies). Some scientists think dark matter is concentrated in huge masses such as black holes.

Our solar system lies within the Milky Way galaxy. The Milky Way galaxy contains over 200 billion stars and has a diameter of up to 120,000 light years. Our solar system orbits around the Milky Way once every 200-250 million years. You can see the Milky Way galaxy on a clear night. It looks like a milky white band across the night sky.