Dark Matter and Dark Energy: What We Actually Know vs. What We Assume

Here's what the Standard Model of physics can account for: about 5 percent of the energy content of the universe.

That's not a typo. Everything we've ever detected, measured, catalogued, or built a theory about — every atom, every photon, every particle in every accelerator experiment — constitutes roughly one-twentieth of what the universe apparently contains. The other 95 percent is stuff we can infer from its gravitational effects but can't see, detect directly, or explain with any theory that's survived experimental testing.

This is either the most embarrassing situation in the history of science or the most exciting, depending on how you look at it. Most physicists opt for excited, if only because "embarrassing" doesn't really describe a problem you don't know how to solve yet.

The problem announced itself gradually. In the 1930s, Swiss astronomer Fritz Zwicky was studying the Coma galaxy cluster — a collection of thousands of galaxies bound together gravitationally — and noticed something odd. The galaxies were moving too fast. Based on the visible mass of the cluster, there wasn't enough gravitational pull to hold them together at those velocities. Either the physics of gravity was wrong at large scales, or there was a lot of matter in the cluster that wasn't emitting any detectable light.

Zwicky called this "dunkle Materie" — dark matter. He was largely ignored for forty years.

The vindication came through Vera Rubin's work in the 1970s on galaxy rotation curves. Stars near the center of a galaxy orbit faster than stars at the edge — you'd expect this, just as Mercury orbits the Sun faster than Neptune. But when Rubin and her colleagues measured actual rotation speeds across dozens of galaxies, they found something strange: the rotation curves were flat. Outer stars were moving almost as fast as inner stars. This only makes gravitational sense if there's a large amount of matter distributed throughout the galaxy in a halo surrounding the visible disk — matter that doesn't emit light but exerts gravitational pull.

By the 1980s, dark matter had become mainstream in cosmology. The question was no longer whether it existed but what it was.

Dark energy entered the picture later and with less warning. In 1998, two independent research teams — both trying to measure the deceleration of the universe's expansion by observing distant Type Ia supernovae — found instead that the expansion was accelerating. This was completely unexpected. Gravity should be slowing the universe's expansion, not speeding it up. Something was providing an outward push. That something, which fits into Einstein's equations as a cosmological constant but has no physical explanation anyone is satisfied with, was called dark energy.

Current best estimates: dark matter constitutes about 27 percent of the universe's energy content. Dark energy accounts for about 68 percent. Normal matter — us, plus everything we can see or touch — is the remaining 5 percent.

The universe, in other words, is mostly made of things we know almost nothing about. This chapter is about dark matter. The next few chapters are about what we actually know versus what we're pretty much guessing. The honest answer is that the ratio isn't as favorable as you might hope.

The Missing 95% — When the Universe Stopped Adding Up

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