Did You Know? Dark Matter & Dark Energy: Why We Only See 5% of the Universe

Sat, 19 Sep 2026 00:00:00 +0000

A headline stopped my thumb mid scroll the other day.. every star and galaxy any telescope has ever photographed adds up to about 5% of what’s actually out there. Not 5% of what we’ve found. 5% of everything. I had to sit with that for a bit.

While we’re on this topic, don’t miss “Did you know? Boiling in space is backwards”:

The other 95% doesn’t emit light, doesn’t reflect it, and barely touches normal matter at all. Cosmologists split it three ways. Normal matter, about 5%, is everything made of atoms — you, me, every planet, every star. Dark matter is about 27%, invisible mass holding galaxies together through gravity. Dark energy is 68%, doing the opposite, pushing space apart a bit faster every year.

Shrink the whole universe down to a person and normal matter wouldn’t even register as a speck of dust on their skin. That’s the part that stuck with me the most.

Stars that wouldn’t slow down

Fritz Zwicky noticed it first, back in the 1930s. Galaxies in the Coma Cluster were moving too fast for their visible mass to hold them together. Nobody did much with it for decades.

Vera Rubin picked it back up in the 1970s. She was measuring how fast stars orbit inside spiral galaxies. Basic physics says stars far from the center should move slower than ones near the middle.. Neptune crawls compared to Mercury, that kind of thing. Rubin found stars way out on the edges moving just as fast as the ones near the core.

Only explanation: there’s way more mass out there than anyone can see, enough extra gravity to keep those outer stars from flying off. Nobody has ever detected that mass directly, not once. You know it’s there the way you know it’s windy by watching trees bend.

Then came the Bullet Cluster. Two galaxy clusters collided, and most of the mass ended up somewhere other than the visible gas.. gravity pointing at something invisible again.

So what actually is it

This is the part that trips people up. “Dark matter” sounds like a substance, like it should be sitting in a jar somewhere in a lab. Nobody knows what it’s made of. The leading guess for decades was WIMPs, weakly interacting massive particles, a particle that would barely ever bump into normal matter, which is exactly why it’d be so hard to find.

There are underground labs built just for this. Places like the Xenon experiment in Italy, buried under a mountain to block out cosmic rays, sitting there waiting for a dark matter particle to occasionally smack into a tank of liquid xenon. Years of waiting.. mostly nothing. WIMPs haven’t shown up where physicists expected them, which pushed some researchers toward other candidates, axions, or the idea that dark matter isn’t a particle at all but some effect of gravity we don’t understand yet.

Nobody’s ruled that last option out completely. Most cosmologists still bet on some kind of particle though. Either way it’s one of the biggest open questions in physics, and the honest answer right now is we don’t know, we’re testing.

The part that gets me

For most of the 20th century physicists assumed gravity would slow the expansion down. The Big Bang flung everything outward, gravity was supposed to be the brakes, maybe even reverse it eventually.

In 1998 two separate teams were measuring distant exploding stars called Type Ia supernovae. These burn at a known, fixed brightness, so how dim one looks tells you how far away it is. Both teams expected to find the expansion slowing. They found the opposite. Speeding up. Not slowing, not steady.. accelerating, faster the longer you watch.

Nobody built a model expecting that. It just showed up in the data and held up. Won the Nobel Prize in Physics in 2011. We still don’t have a real explanation for it, “dark energy” isn’t a real description, it’s just a name for “we don’t know.”

DESI, an instrument mapping millions of galaxies, has recently hinted dark energy might not even be constant. It could be weakening over time. Its March 2025 analysis drew on three years of data covering nearly 15 million galaxies and quasars, combined with studies of the cosmic microwave background, supernovae, and weak gravitational lensing. Not settled yet, but if it holds up, that’s a real headache for the standard model of cosmology.

Who’s looking

Webb wasn’t built for this specifically, but its view of extremely old, distant galaxies is filling in how structure formed under dark matter’s influence. Already reshaping timelines that were based on older, blurrier data.

Euclid, launched by the European Space Agency in 2023, was built specifically for this, mapping dark matter’s distribution and dark energy’s effects across billions of light-years. It’s slowly building the largest 3D map of the universe ever made.

Roman launched too, on a Falcon Heavy, August 30, 2026. Its field of view is about 100 times larger than Hubble’s. It’ll spend its five year mission surveying billions of stars and galaxies, hunting exoplanets, trying to pin down what dark energy is actually doing. Between Roman, Euclid, and DESI, this next decade might finally produce enough data to say something definite, or it might just deepen the mystery.. wouldn’t shock me either way.

Dark matter pulls things together. Dark energy pushes them apart. That’s really the only clean line you can draw between them, everything else about both is still a shrug dressed up in a name.

I keep telling myself I’ll fully wrap my head around this stuff eventually.. still haven’t.

Cosmology on What Just Happened NOW

Did You Know? Dark Matter & Dark Energy: Why We Only See 5% of the Universe

Stars that wouldn’t slow down

So what actually is it

The part that gets me

Who’s looking

Further reading (see source below)