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A Galaxy can be roughly defined as a collection of very many stars (ten million stars to a million times a million) stars bound together by Gravity. Our sun is in a galaxy that is called the "Milky Way" and a relatively nearby galaxy is the Great Andromeda Galaxy.
Galaxies vary in size and shape, some are elliptical , some are spiral  and some are irregular . Galaxies contain Dark matter, stars, planets, black holes, interstellar gas and dust as well as other objects like comets. New stars form from the gas and dust in a galaxy. Astronomers believe that at the centre of every galaxy is one terrifyingly large black hole.
Light from distant galaxies has been traveling for millions of years, hundreds of millions of years and thousands of millions of years to reach Earth. The existence of light that has been traveling for a very long time is yet more evidence against Young Earth Creationism, this is known as the starlight problem.