This Is How Our Earliest Picture Of The Universe Shows Us Dark Matter

If you go all the way back to where neutral atoms first formed, you can see the cosmic microwave background. Buried in the details is the Universe’s first evidence for dark matter.

One of the greatest mysteries of modern science is the puzzle of dark matter. If you add up all the normal matter making up planets, stars, gas, plasma, black holes, galaxies, and the space between galaxies — all the matter in the known Universe — it isn’t enough to explain the gravity we see. It can’t explain individual galaxies, clusters of galaxies, colliding groups of galaxies, gravitational lensing, or the large-scale structure of the Universe. Something more must be out there, and it can’t be normal matter.

The name we’ve given this mysterious substance is dark matter. Dark because it doesn’t interact with light or normal matter; it can’t be seen. Matter because it gravitates, clumps, and clusters together. Although there’s a controversy over exactly what dark matter is, its existence is virtually certain, as it shows up in every astronomical observation possible. Even, as we discovered earlier this century, in the earliest picture of the Universe we could ever take: of the Big Bang’s leftover glow.

Billions of years ago, closer back in time to the Big Bang, the Universe was denser and more uniform. It takes billions of years to form the large galaxy clusters we see today, hundreds of millions to form the first galaxies, and tens of millions to form the first stars. Because an expanding Universe also cools — the energy of any individual photon is proportional to its wavelength, and all “lengths” stretch (to lower energies) as the Universe expands — the early Universe was not just smaller, but also hotter. At some point in the past, the Universe was hot enough that every neutral atom that formed, every electron bound to an atomic nucleus, would be dissociated into free ions by the radiation that was created in the hot Big Bang.

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