Astronomers trying to piece together the universe's past have two major pieces of evidence with which to work. The first is that the whole thing began with a Big Bang, an explosion of unimaginable heat and power, between 10 billion and 20 billion years ago. The second is that the modern-day cosmos is made up of galaxies. Gravity presumably played a role in the process, but the details are unknown.
For the past decade or so, the best scientific guess about the evolution of the universe has been the cold-dark-matter (CDM) theory, which holds that an exotic, unseen form of matter...
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