Dark energy, however, is not the only “dark” oddity discovered in recent times. Another was confirmed in the 1980’s when astronomers examined various galaxies. These galaxies, as well as our own, appeared to be spinning too fast to hold together. Evidently, then, some form of matter must be giving them the necessary gravitational cohesion. But what kind of matter? Because scientists have no idea, they have called the stuff dark matter, since it does not absorb, emit, or reflect detectable amounts of radiation.c How much dark matter is out there? Calculations indicate that it could make up 22 percent or more of the mass of the universe.

Consider this: According to current estimates, normal matter accounts for about 4 percent of the mass of the universe. The two big unknowns​—dark matter and dark energy—​appear to make up the balance.

[Diagram on page 18]

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74% dark energy

22% dark matter