## The darkness of dark matter and dark energy

ALL THAT WE KNOW of the universe we get from observing photons, Natarajan pointed out. But dark matter, which makes up 90 percent of the total mass in the universe, is called dark because it neither emits nor reflects photons — and because of our ignorance of what it is. It is conjectured to be made up of still-unidentified exotic _collisionless_ particles which might weigh about six times more than an electron.

Though some challenge whether dark matter even exists, Natarajan is persuaded that it does because of her research on “the heaviest objects in the universe“ — galaxy clusters of more than 1,000 galaxies. First of all, the rotation of stars within galaxies does not look Keplerian — the outermost stars move far too quickly, as discovered in the 1970s. Their rapid rate of motion only makes sense if there is a vast “halo” of dark matter enclosing each galaxy.

And galaxy clusters have so much mass (90 percent of it dark) that their gravitation bends light, “lenses” it. A galaxy perfectly aligned on the far side of a galaxy cluster appears to us — via the Hubble Space Telescope — as a set of multiple arc-shaped (distorted) galaxy images. Studying the precise geometry of those images can reveal some of the nature of dark matter, such as that it appears to be “clumpy.” With the next generation of space telescopes — the James Webb Space Telescope that comes online in 2018 and the Wide-Field Infrared Survey Telescope a few years afterward — much more will be learned. There are also instruments on Earth trying to detect dark-matter particles directly, so far without success.

As for dark energy — the _accelerating_ expansion of the universe — its shocking discovery came from two independent teams in 1998–99. Dark energy is now understood to constitute 72 percent of the entire contents of the universe. (Of the remainder, dark matter is 23 percent, and atoms — the part that we know — makes up just 4.6 percent.) When the universe was 380,000 years old (13.7 billion years ago), there was no dark energy. But now “the universe is expanding at a pretty fast clip.” Natarajan hopes to use galaxy-cluster lensing as a tool “to trace the geometry of space-time which encodes dark energy.”

These days, she said, data is coming in from the universe faster than theory can keep up with it.” We are in a golden age of cosmology.”