Question

What is a group of stars, gas and dust held together by gravity called?

Answer 1

Hint: They are lying systems of dust, gas, dark matter and trillions of stars are held together by gravity. It is thought that they have supermassive black holes at their centres. They may occur alone or in pairs. They are part of associations which are known as groups, clusters and super clusters. It is believed that they are formed from smaller clusters of about one million stars.

Complete answer:
Galaxies held together the group of stars, gas and dust. They have varied shapes and sizes. Galaxies have dust which are produced by stars, it causes light that appears redder than it is when observed visually.
Our galaxy's milky way has barred spiral galaxies. Our galaxy's milky way has hundreds of billions of stars. Our galaxy holds ten times as much dark matter if all the stars and gas are put together. And all of this is held together by gravity.
 It takes 250 km/sec and it completes a full revolution every 200 million years. It is concluded that extra gravitational force comes from dark matter. Dark matter does not emit or reflect light.

Note:
Our galaxy is named milky way because it appears to be a milky band of light in the sky. It is estimated that there are about 100 billion galaxies in the universe. There are two closest galaxies to the milky way which are the small Magellanic cloud and the large Magellanic cloud. These two may have no black holes because they are too small to detect. If we look in the cosmos, we will find more galaxies. Galaxies rotate faster than the stars based on the gravity of their stars.