# Dark Matter Question

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I understand that astronomers often locate planets orbiting other stars. They do this by detecting the "wobble" of a star. They can't actually see the planet right? Isn't it true that dark matter is also detected by it's gravitational influence on nearby objects? Maybe this is a dumb question, but how do we know that the dark matter isn't just stuff like planets that we can't see because stars are in the way, or are too bright?

Last edited: Sep 20, 2009
2. ### Janus

2,524
Staff Emeritus
Look at it this way. In our solar system, planets and such only make up about 1/10 of 1% of the mass. The rest is the Sun.

There should be 5 times as much Dark matter as regular matter to explain the observations. That's the equivalent of our Solar system having 5000 times as much matter than it does now in the way of planets and such.

That much extra normal matter should be noticeable even if just by the dimming effect it would have on the light we see from the stars.

52
I think there is more to AdkinsJr question than what Janus responded to.

In principle Dark Matter (DM) could be in the form of planets or dim stars (e.g. brown dwarfs) or black holes. Because the average distance between these objects would be so large,you won't always see starlight dim (unless a DM object eclipses a star).

My guess is that people have ruled out brown dwarfs and planets as being DM for two possible reasons. First off, planetary and stellar formation theories don't allow for most of the matter in a galaxy to collapse into brown dwarfs and planets. Second, someone may have done a direct search for brown dwarves that showed there aren't enough to explain dark matter.

A much more respectable possibility is that massive compact objects (e.g. neutron star or black hole) comprise DM. Astronomers can search for these compact objects through microlensing effects. When a compact object passes in front of a star the compact object's gravity can act like a lens and magnify the stars light for us. So by keeping our eye (i.e. telescope) on a bunch of stars we can look for spikes in their brightness which could be these compact objects. This type of survey has been carried out and has ruled out DM being black holes with a mass of $10^{-8}M_{sun}$ (according to the wikipedia entry on this subject).

Thus, most astronomers think that DM is comprised of particles that lie outside our current understanding of particle physics.

4. ### mathman

6,751
One additional factor against dark matter being baryonic is the observed ratio (big bang origin) of H1, H2, He3, He4, Li6, Li7 is consistent with the known baryonic density in the universe. Dark matter, if baryonic, would increase the density by a factor of six, therefore not consistent with the ratios describe above.

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Mathman,

If dark matter were comprised of primordial black holes that were present before the time of big bang nucleosynthesis, this would not contradict the primordial elemental abundance results. Right?

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Most scientists seem to accept dark matter, but there are some skeptics. I have heard about an alternative to general relativity, called the "asymmetric theory of gravity," which does away with dark matter and results in dramatically different cosmological model.
However, if general relativity holds, and dark matter behaves accordingly, wouldn't dark matter bend light? Has this ever been detected?