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Q & A: Dark Matter

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Most recent answer: 10/22/2007
Q:
Does Black Matter exsist if so what does it consiste of?
- Steven Henshall (age 16)
Catoosa High School, Claremore,Oklahoma, United States
A:
Good question! We have lots of reasons to believe that matter which is made up of atoms makes up only a tiny fraction of the total matter and energy in the universe -- about 4% according to current estimates. There are lots of measurements that confirm this.

One of the older ones which is quite compelling is just looking at how fast stuff is going as it orbits the centers of galaxies. Most of the stars in a galaxy are near the core, and the density of stars falls off as you get farther away from the core of a typical galaxy. Even if we don't understand what makes up the matter of the core (stars, black holes, what have you), you can still predict how the rotation speed will vary with the distance from the core, because we can estimate how much stuff there is outside the core, in the arms. Then we can measure the rotation speed everywhere in a galaxy by looking at the Doppler shifting of the light coming back. What we get doesn't match with the assumption that the matter is in the core and in visible matter in the arms/disks of galaxies. It's a big deal, too -- the estimations are off by a factor of four or more for most galaxies. Dark, unseen matter clumping around galaxies would help solve the problem.

Also, measurements of the large scale structure of galaxies and clusters of galaxies indicates that additional matter must be present to provide the gravitational attraction to clump the matter on the time scales from the beginning of the universe until now. Measurements of the cosmic microwave background and supernovas give very precise estimates of the age of the universe and the cosmic microwave background also tells us how clumpy the matter was shortly after the big bang. It all makes sense if there is about four to five times as much dark matter as matter which we can see.

It gets worse. The expansion of the universe appears to be speeding up (again, the cosmic microwave background and supernova experiments are important)! To explain this, we need to introduce another component of the matter and energy balance of the universe. About 73% of everything is required to be a negative-pressure, "dark energy" stuff. 23% of everything is dark matter, and 4% is the familiar atoms we know and love.

We don't know what stuff makes up the dark matter and we wish very much that we did. It's not neutrinos, because these zip away from galaxies and don't orbit them. Whatever the dark matter is, it doesn't interact with ordinary matter very much or at all (except through gravitational attraction). We hope that it interacts via the weak nuclear force so we have a chance of detecting it in laboratory experiments. Some hypotheses of what dark matter is are supersymmetric partners of ordinary, known particles (the partner of the photon and weak force carriers is an attractive candidate for a dark matter constituent), or axions (another neutral, weakly-interacting hypothetical particle), or perhaps anything else exotic you can think of. I have no idea what the dark energy consists of.

Tom

(published on 10/22/2007)

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