Читать книгу Universe: The story of the Universe, from earliest times to our continuing discoveries - Peter Grego - Страница 20

Only a century ago most astronomers thought that the Milky Way represented the entire Universe. We now know that the Milky Way is just one of billions of other galaxies – some larger, some smaller than our own. The nearest big galaxy to the Milky Way is the Andromeda Galaxy, 2.9 million light years away, a spiral similar to our own. Discernable with the unaided eye on a dark, clear night, the light from that ghostly oval smudge in Andromeda set off long before the first sparks of consciousness flickered within the minds of human beings.

Telescopic surveys have shown us the structure of the wider Universe. Galaxies are arranged in gravitationally-bound clusters and superclusters, immersed in vast clouds of gas. Incredibly, the matter that can be observed telescopically – planets, stars, interstellar gas and dust clouds – make up a small proportion of the matter in the Universe. A staggering 90 per cent is thought to take the form of ‘dark matter’, currently unable to be detected by any telescope. This mysterious stuff is known to exist because its mass produces a detectable gravitational pull on galaxies. What constitutes dark matter is a subject of much debate among astronomers. It may be an entirely new form of matter, quite unlike the stuff we are made of, or it may simply be as yet unobserved ordinary matter such as old failed stars known as ‘brown dwarfs’, or more exotic entities like black holes.