Universe means unimaginably vast space which is unbounded with its dots of body's are by definition, the universe includes everything that abounds in space, and space is filled with matter which varies from the tiniest and cosmic particle to the gigantic galaxies. The universe is known to composed of many types of comparatively small and large dense Bo e.g., the galaxies, the constellations (star groups), the stars, the planets other heavenly bodies.
The totality of the heavenly bodies from grand galaxies down to the sm cosmic dust filling the endless space describes the Universe w dimension is beyond our comprehension.
There are t divergent theories on the mode of origin of the universe. These are the B Bang and the Steady State theories. According to 'Big Bang' theory universe has a finite beginning, through a tremendous explosive expansion superdense and very condensed matter. The "steady state" theory states the universe expands through a process of continuous creation of matt which (the process) remains unchanged through eternity.
Galaxy: A galaxy is a rotating stellar system (star system) consisting of a swarm of stars which are held together by mutual gravitational attraction. In other words, a galaxy is an enormously large cluster of stars. Our galaxy to which the sun along with its planet families belong is an aggregate of aboii one hundred billion stars. Ours is called "Milky way" galaxy which looks like a discuss (a disc-shaped metal used in discus throw) when viewed through powerful optical telescope.
It is a pancake shaped whirling body which is gigantic in size. A part of the "Milky way" is clearly seen with naked eye as a faint luminous broad band across the sky during a clear autumn night. It takes about 100,000 years for the light to go from one end of our galaxy to the other end. There are billions of galaxies in the universe. The nearest galaxy comparable to our Milky Way in size is the Andromeda galaxy.
Stars and Planets: A star is a self-luminous and incandescent globe of hot gases. Stars produce their own light and are seen twinkling in the sky. The stars vary in size, temperature and constituent materials. Many stars occur in pairs (twins). They may even occur in triplets or quadruplets.
Most of the stars have their own systems of planets and satellites revolving round them. In such cases, a star constitutes a star system. Our sun is a star having its own planetary and satellite system called the "solar system".
Planets and satellites are non-luminous heavenly bodies which shine only because of reflection of star (sun) light from their surface. The planets whirl round their stars in definite orbits.