Human race despite its greatest science inventions has not been able to resolve several mysteries created by God one of which is the outer space the most of which is still hidden from human eye.
Our universe is believed to be made up of various small and large distinct gravitationally bound space systems made up of dust, gas and stars known as galaxies. The word galaxy is derived form greek word "Galaxias" which mean "milky circle" and hence we know our own Galaxy as Milky way". The galaxies are so unbelievably gigantic that galaxies having less than one billion stars have been classified as small galaxies and to your information our SUN is just one of those billion stars existing in our own galaxy Milky Way. We know that the system in which various planets orbit the sun is known solar system. Small Galaxies known as dwarf may have about 10 million stars whereas large galaxies can go upto a hundred trillion stars. These stars revolve around galaxy's center of mass. Large numbers of galaxies are arranged into a hierarchical association known as clusters. Clusters forming large groups are called super clusters. About 90% mass of all galaxies is accounted for by Dark Matter which is another mystery not yet resolved by mankind. It is also believed that black holes may exist at the center of most of the galaxies.
The distances within space are mind boggling as these are counted in light years. One Light year is the distance which can be traveled if one travels with speed of light for full one year. The galaxy closest to own Milky way is known as Andromeda Galaxy which is two million light years away from us. So far we have been able to see a total of 100 billion galaxies whereas it is not possible to count all the galaxies. Distance between two galaxies is known as intergalactic space.
It is believed that galaxies owe their existence to the famous BIG BANG which occurred about ten to twenty billion years ago and is believed to be the starting point of universe. After the explosion of Bing Bang gigantic cloud of Gas began to collapse and eventually formed these building blocks of the universe.