Dark Matter and Dark Energy

Imagine weighing a house and finding that the furniture, walls and people account for only one twentieth of the total. The rest is something you cannot see, cannot touch and cannot find anywhere inside. That is the situation astronomers have been in for decades. Add up all the stars, gas, dust and planets in the universe and you get about 5 percent of what the universe appears to weigh. The other 95 percent is dark.

Galaxies spinning too fast

The first hint came in the 1930s, when the astronomer Fritz Zwicky noticed that galaxies in a cluster were moving so fast they should fly apart. Something unseen had to be holding them. The decisive evidence arrived in the 1970s, from Vera Rubin and Kent Ford. They measured how fast stars orbit at different distances from the centre of spiral galaxies.

In the solar system, planets farther from the Sun move more slowly, because the Sun's pull weakens with distance. Galaxies should behave the same way, with stars at the edges crawling along. Instead, Rubin found that the outer stars move just as fast as the inner ones. The only explanation within our understanding of gravity is that galaxies are embedded in enormous halos of invisible mass, five or six times heavier than everything we can see.

What dark matter is not

Dark matter is not simply ordinary stuff that happens to be dim. Astronomers have ruled out dead stars, rogue planets, black holes and cold gas; there is not nearly enough of any of them, and their gravity would bend light in ways we would have noticed. The pattern of the cosmic microwave background, the afterglow of the Big Bang, shows that most of the matter in the universe was already something that did not interact with light when the universe was only 380,000 years old.

Whatever dark matter is, it feels gravity, it ignores light, and it barely touches ordinary matter at all. The leading idea is that it consists of a new kind of particle, slow-moving and heavy, left over from the Big Bang. Detectors buried deep underground, shielded from everything else, have been waiting for one of these particles to bump into an atom for over thirty years. So far, nothing.

The universe speeding up

In the 1990s two teams set out to measure how fast the expansion of the universe was slowing down under the pull of all its matter. They used exploding stars of a particular kind, type Ia supernovae, which always reach nearly the same peak brightness and so serve as distance markers. The result shocked everyone. The expansion was not slowing. It was speeding up.

Something is pushing space apart, and it gets stronger as the universe gets bigger. We call it dark energy, which is less a name than an admission of ignorance. It appears to be a property of space itself: every cubic metre of empty space carries a tiny amount of energy that resists gravity. As space expands there is more of it, so the push grows. Dark energy now accounts for about 68 percent of the universe's total energy, and its share is rising.

The cosmic recipe

Combining measurements of supernovae, the cosmic microwave background, the clustering of galaxies and the bending of light by massive clusters, cosmologists arrive at a consistent recipe. About 5 percent of the universe is ordinary matter. About 27 percent is dark matter. About 68 percent is dark energy. Different methods, measuring different things, land on the same numbers, which is why most scientists are confident the dark components are real even though nobody knows what they are.

Why it matters

Dark matter is the scaffolding of the cosmos. Without its extra gravity, the faint ripples in the early universe would never have collapsed into galaxies in time; there would be no Milky Way and no Earth. Dark energy, meanwhile, decides the future. If it keeps pushing, galaxies beyond our local group will eventually recede faster than light can cross the gap, and the sky will slowly empty. In a trillion years an astronomer in our galaxy would see nothing beyond it.

What we still don't know

Almost everything. We do not know what dark matter is made of, or whether it is a particle at all; a few physicists argue that the laws of gravity themselves need modifying. We do not know whether dark energy is truly constant or changes with time, which would completely alter the universe's fate. The two greatest mysteries in physics are not exotic details. They are 95 percent of everything.