What Are Dark Matter and Dark Energy?
The short answer
Everything ever observed (every star, planet and galaxy) adds up to only about 5% of the universe. The rest is invisible. About 27% is dark matter, which holds galaxies together. About 68% is dark energy, the unknown push that makes the expansion of the universe speed up.
We know about both from what they do, not from seeing them. Dark matter shows up through its gravity. Dark energy shows up in the way distant galaxies pull away faster and faster.
How we know dark matter is there
Vera Rubin's measurements showed that stars near the edge of a spiral galaxy orbit about as fast as stars near the center. The gravity of a galaxy's visible stars alone couldn't hold it together at that speed. It should have torn itself apart long ago. Something invisible adds the missing gravity, and there is roughly five to six times as much of it as normal matter.
Dark matter doesn't give off, absorb or reflect light, and it barely touches ordinary matter. If it is made of particles, which is the leading idea, billions of them pass through your body every second. Almost none of them ever hits anything.
Faster than light, and still legal
In 1998 astronomers found that the expansion of the universe is speeding up. The discovery came from distant supernovae and won the 2011 Nobel Prize in Physics. Nobody knows what drives the speed up. "Dark energy" is the name for that unknown push. It began winning about 5 to 6 billion years ago.
Galaxies beyond about 14 billion light years are moving away from us faster than light. That doesn't break relativity, because they aren't moving through space that fast. The space between us and them is growing. We can even see some of these galaxies, by light they sent long ago.
One galaxy left
Our neighborhood, the Local Group, includes the Milky Way and Andromeda. Gravity holds it together, and it will merge into one large galaxy. Almost everything beyond it is being carried away. In about 100 billion years, an astronomer in that merged galaxy would see just one galaxy in the sky: their own.
Key numbers
| What | Number | Source |
|---|---|---|
| Share of the universe that is normal matter (all stars, planets and galaxies) | about 5% | NASA Science, Dark Matter |
| Share of the universe that is dark matter | about 27% | NASA Science, Dark Matter |
| Share of the universe that is dark energy | about 68% | CERN, Dark matter |
| Dark matter compared with normal matter | about 5.4 times as much (27% ÷ 5%) | Our arithmetic from NASA Science, Dark Matter |
| Dark matter particles passing through a human body, if dark matter is made of particles | billions per second | Freese & Savage 2012, Physics Letters B |
| Year astronomers found the expansion is speeding up | 1998 | NASA Science, Hubble Finds Evidence for Dark Energy in the Young Universe |
| Nobel Prize for discovering the speeding up expansion | 2011 (Physics) | Nobel Prize in Physics 2011 |
| When the expansion began speeding up | about 5 to 6 billion years ago | NASA Science, Hubble Finds Evidence for Dark Energy in the Young Universe |
| Distance beyond which galaxies recede faster than light | about 14 billion light years | Davis & Lineweaver 2004, PASA |
| Time until nearly all other galaxies are gone from view | about 100 billion years | Krauss & Scherrer, Scientific American 2008 |
What the film simplifies
- The film says "less than five percent". NASA and CERN both give about 5% for all normal matter, meaning every star, planet and galaxy.
- "Probably through you, right now" is only true if dark matter is made of particles. That is the leading idea, but nobody has proved it yet.
- Dark matter's hug, its face and its name tag are cartoon. What actually holds the galaxy together is gravity.
- When the invisible mass splits in two, the film draws the dark matter half bigger than the dark energy half. It's the other way round: about 27% against 68%.
- The race between a beam of light and the growing gap can suggest we never see galaxies that are receding faster than light. In fact we see some of them today, by light they sent long ago.
- "Ours" in the last line means the galaxy we will live in by then: the Milky Way merged with Andromeda and the rest of the Local Group.
For teachers
Runtime 1 minute 5 seconds · Topics: Dark matter, Dark energy, Expanding universe, Galaxies · Full script below
Ask your class
If dark matter gives off no light, how do we know it's there?
Answer
From its gravity. Galaxies spin so fast that the gravity of their visible stars couldn't hold them together, so something unseen must add the extra pull.
The farthest galaxies are moving away from us faster than light. Why doesn't that break the cosmic speed limit?
Answer
They aren't moving through space that fast. The space between us and them is growing, and the speed limit only applies to motion through space.
Dark matter is about 27% of the universe and normal matter about 5%. Roughly how many times more dark matter is there?
Answer
About 5 times (27 ÷ 5 = 5.4), so dark matter is roughly five to six times as plentiful as normal matter.
The script
Host: Everything you've ever seen is less than five percent of the universe.
Rocket: Five percent?! But I planted a flag!
Host: The rest is invisible. One part holds us together. The other is emptying our sky.
Host: Our galaxy spins so fast, it should fling its stars into space.
Lil Sat: Then why aren't we flying off?!
Host: Because something invisible is hugging it tight. We call it dark matter.
Rocket: Hold my fuel. I'll catch it!
Host: Nope. It passes right through things. Probably through you, right now.
Host: The other part pushes. It's making space stretch faster and faster.
Lil Sat: Stop pushing! What even is that?!
Host: No idea. That's why we call it dark energy.
Host: And it's winning. The farthest galaxies are being carried off faster than light.
Rocket: Faster than light?! That's cheating!
Host: Totally legal. They're not flying. The space in between is growing that fast.
Host: In about a hundred billion years, our sky will show one galaxy. Ours.
Rocket: Wait. Then who's gonna watch me fly?!
Host: So if we ever move, it'll be somewhere close. Like Mars? Follow!
Sources
- Everything ever observed adds up to less than 5% of the universe.NASA Science, Dark Energy
- Dark matter makes up about 27% of the universe.NASA Science, Dark Matter
- Vera Rubin found galaxies spin too fast for their visible mass.Smithsonian American Women's History Museum
- If dark matter is particles, many pass through you every second.Freese & Savage 2012, Physics Letters B
- The expansion of the universe is speeding up.Nobel Prize in Physics 2011
- The most distant galaxies recede faster than light as space expands.Davis & Lineweaver 2004, PASA
- In about 100 billion years, other galaxies will vanish from view.Krauss & Scherrer 2007
- Normal matter is 5% of the universe, dark matter about 27%, dark energy about 68%; galaxies spin too fast for their visible matter to hold them together; dark matter does not absorb, reflect or emit light.CERN, Dark matter
- In 1998 astronomers found the expansion is speeding up; it began speeding up about five to six billion years ago.NASA Science, Hubble Finds Evidence for Dark Energy in the Young Universe
- The Local Group will collapse into one large galaxy over about 100 billion years, and all other galaxies will disappear beyond the event horizon.Krauss & Scherrer, Scientific American 2008