An image of the cosmic microwave background doesn’t necessarily look like much, but the data it contains speaks volumes about the beginning of the universe.
Courtesy Shaul Hanany

The CMB is also answering questions about what the universe is made of. Scientists noticed decades ago that all the matter and energy in the universe doesn’t seem to account for some cosmic behavior—the universe is expanding faster than all observable energy should allow, and galaxies move in ways that require more matter than they seem to have. So astronomers have proposed that most of the matter and energy in existence is “dark.” Specifically, what we can see and measure may only be about 4% of what’s out there. But because we can’t directly observe dark matter, this has been hard to demonstrate—it’s like describing something hiding underwater after looking at only the swirls and ripples it makes. The size and distribution of CMB fluctuations (the universe’s “swirls and ripples”), however, match up perfectly with previous dark matter estimates. It’s strong evidence that 96% of all the matter and energy in the universe is dark, and totally invisible to us.

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