the nature of dark matter and dark energy

Add up everything astronomers can see — every star, planet, gas cloud, and glowing galaxy — and you have accounted for only about five percent of what the universe is made of. The other ninety-five percent is missing from our inventory of known particles. Roughly a quarter of it is dark matter, an unseen substance whose gravity holds galaxies together, and the remaining seventy percent is dark energy, a smooth something that is pushing the universe to expand faster and faster. We know they are there only by their pull and their push; we do not know what either one is.

Dark matter and dark energy are two very different mysteries that happen to share the word 'dark,' meaning 'we cannot see it.' Dark matter behaves like ordinary matter in one key way — it clumps under gravity — but it gives off no light and seems to feel neither electromagnetism nor the strong force, so it has slipped past every detector built to catch known particles. Its existence is inferred from how fast stars orbit the edges of galaxies, from how galaxy clusters bend background light, and from the detailed pattern of the cosmic microwave background. Dark energy is stranger still: it does not clump at all, it fills space evenly, and instead of pulling things together it pushes them apart, which is why distant galaxies are receding ever faster.

For particle physics this is the most direct evidence that the Standard Model is incomplete: it simply contains no particle that can be the dark matter, and no field that naturally behaves like dark energy. The leading guesses for dark matter are new particles — weakly interacting massive particles, or axions, or particles from a hidden sector — and experiments hunt for them by waiting for a rare bump against an underground detector, by looking for their faint annihilation glow in the sky, or by trying to make them at colliders. Dark energy is even less understood and is often described simply as a cosmological constant, an intrinsic energy of empty space, whose tiny measured value is itself a deep puzzle.