If the universe started out as a dense "soup" of particles of matter and energy, then how did it get "lumpy" enough for galaxies to form? Did they start out as a huge clouds of gas that contracted - or, instead, as small clumps of stars that merged?
Even more puzzling, how did galaxies coalesce into vast clusters?
The largest such structure, called the "Great Wall" of galaxies, is nearly 500 million light-years long. According to current theories, the universe simply hasn't been around long enough to build such unimaginably vast structures.
1 idea is that the universe contains infinitely long but infinitesimally thin "cracks" called cosmic strings. Like crystal growing on a piece of thread, matter might have accumulated along these strings early in the universe's history to eventually form great filamentary clusters of galaxies. There is no direct evidence for cosmic strings as yet, however.
A more widely accepted possibility is that galaxies may just be "puddles" or "trace sediments" in a "haze" of invisible dark matter that may have been clumpy before galaxies formed. This material was first suspected when astronomers measured its ghostly gravitational pull on galaxies within distant cluster.
Today, astronomers believe that this dark or missing matter may account for up to 90% of the mass of the universe. Since it is invisible, its identity remain unknown. Scientist have proposed possibilities that range from subatomic particles to black holes.
If the universe has enough dark matter, its gravitational pull will eventually slow the expansion of the universe, and pull it back onto itself. If that were to be happen someday, all matter would be compressed into a super-dense fireball similar to that from which the universe emerged 15 to 20 billion years ago.
1 idea is that the universe contains infinitely long but infinitesimally thin "cracks" called cosmic strings. Like crystal growing on a piece of thread, matter might have accumulated along these strings early in the universe's history to eventually form great filamentary clusters of galaxies. There is no direct evidence for cosmic strings as yet, however.
A more widely accepted possibility is that galaxies may just be "puddles" or "trace sediments" in a "haze" of invisible dark matter that may have been clumpy before galaxies formed. This material was first suspected when astronomers measured its ghostly gravitational pull on galaxies within distant cluster.
Today, astronomers believe that this dark or missing matter may account for up to 90% of the mass of the universe. Since it is invisible, its identity remain unknown. Scientist have proposed possibilities that range from subatomic particles to black holes.
If the universe has enough dark matter, its gravitational pull will eventually slow the expansion of the universe, and pull it back onto itself. If that were to be happen someday, all matter would be compressed into a super-dense fireball similar to that from which the universe emerged 15 to 20 billion years ago.
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