Hmm, that explains a question I have often wondered about. How much can dark matter affect stellar processes? Naively I had though that one would expect some reasonable amount of dark matter to reside inside a star, and therefore, if in no other way, affect the rates of stellar processes though slightly increased gravitational pressure over what you would expect from just the baryonic matter present. I guess the answer is that whilst there may be a bit, there isn’t enough to matter.
Bazinga! 
One thing to keep in mind is that ordinary matter is mostly empty space. So why does a bat hit a baseball and not just sail through it? Or why can I sit in this chair and type? Because the electron clouds repel each other. Ordinary matter clumps because of interactions between one atom’s electron cloud and the next one’s positive nucleus caused by its having lost one or more electrons. Since WIMPS, whatever they are, do not feel the electric field, they interact only through very weak forces, weak interactions and gravity.
I assume, incidentally, that we see neutrinos only because of some missing momentum in such reactions as neutron decay. For they feel only gravity and the weak force. Is that correct?
People have tried various different theories that rejigger General Relativity to account for the effects of dark matter. The problem is there are numerous types of observations that give evidence for dark matter, and so why rejiggering gravity to be consistant with one is easy, doing it for all observations at once becomes pretty implausibly complicated pretty fast.
There are neutrino experiments that detect neutrinos by detecting the nuclear reactions they undergo due to the weak force. The cross-section for these experiments are very low, so you need a very big detector (in one case, they’ve drilled down into the antarctic ice sheet so that they can use a cubic kilometer of ice as the “target” for the detector).
There have been attempts to use the weak interactions of proposed dark matter candidates to use similar experiments to detect dark matter. To many peoples suprise, these experiments have come up empty, suggesting darkmatter must have a much smaller weak interaction then is predicted by many particle theories.
Or just a different kind of weak interaction. Every particle (both the ones we know about and the ones proposed to explain dark matter) interacts through different processes, and so the dark matter detectors generally only look for one proposed kind of dark matter at a time. Sometimes they’re even more specific, and can (for instance) only detect one particular kind of hypothetical particle, and only at one particular energy. Since there are so many proposals for what the dark matter is, we’ve really only just scratched the surface on ruling them out.
Since this is the currently ongoing thread on dark matter, I’ll just leave this here:
Phd Comics’ Illustrated Review of Dark Matter, as informed by Cham’s interview with two physics professors.
That was great!
I don’t get it. If it doesn’t interact, except through gravity, why does it clump at all? Sound like each particle should be on it’s own.
Also, in the comic linked by Spacial Rift, it has a pie chart stating that known matter accounts for 5% of all matter, dark matter an additional 25%, and 70% is completely unknown. I thought dark matter was supposed to be responsible for all the mass that we can’t currently see.
Sinaptics, gravity makes things clump. This is true for “regular” (jargon: baryonic) matter, as well as dark matter. If I have a vast expanse of empty space, and I put two pieces of dark matter in this space, they will be attracted towards each other. That is true for any size pieces of dark matter you want to use, including whatever fundamental particles it might be made of.
As to the pie chart, there’s an additional distinction to be made between matter and energy. Cosmologists define matter as anything that A) has rest mass, and B) exerts very little pressure compared to its density. The current best estimate of the matter in the Universe is that it’s about 16% regular matter, and 84% dark matter. The current best estimate of all the contents of the universe is that 70% is something labeled Dark Energy, 25% is dark matter, and 5% is regular matter (plus trace amounts of electromagnetic radiation). Dark energy is just a placeholder name - we don’t know what it is. We do know that it seems to be exerting an awful lot of pressure in a very special way, which means that when we find out what it actually is, it’s unlikely to be similar to regular or dark matter.
Okay, back to the comic, is uses the example of two galaxies colliding and the dark matter passing through in both directions.
Wouldn’t dark matter being affected by gravity
A. Seek rest in the larger new galaxy since it was apparently tagging along with the old galaxies?
or
B. Have the two clumps of dark matter collect into a new larger clump?
or is it’s linear momentum enough to overcome the force of gravity to keep it traveling along it’s path since it’s not interacting with the other forces. And if so, why would it be tagging along with a galaxy in the first place?
You have the right idea with linear momentum. Both of the colliding galaxies have lots of momentum towards each other. That goes for their respective dark matter clumps, too. When the baryonic matter collides, it actually collides - electrostatic forces are exchanged and it sticks together. The dark matter is still interacting gravitationally through all of this, being pulled towards the new, larger clump of baryonic matter. But there is no electrostatic force to slow it down. It’s like the dark matter doesn’t obey friction. So it zips right through. Remember, now, we have two zipping clumps of DM, one on either side:
DM-A … New Galaxy … DM-B
DM-A and DM-B have momentum away from the center, but they also each have two things pulling them back towards the center gravitationally: the new galaxy and the other clump of dark matter. What will likely happen is they’ll eventually reach a turning point where their existing outward momentum is canceled by the gravitational force, and they’ll fall back in towards the center. Then they’ll zip through again, and we’ll have:
DM-B … New Galaxy … DM-A
Notice I used fewer spaces this time. The more time the two clumps spend in the center, the more time they have under the influence of a stronger gravitational field (force of gravity falls off as the square of distance, remember), so they don’t get as far out as they did before. Eventually the two clumps will settle down and stay put, surrounding the new galaxy.
It’s also possible that DM-A and DM-B already have too much momentum to be held in. Then they’ll fly off into the intergalactic void and never be seen again.
Gotcha. Makes perfect sense. Thanks.
Except how are the lumps of dark matter supposed to orbit around the new merged galaxy more closely over time? The dark matter is just going to go in a hyperbolic or elliptical orbit around the galaxy. If it’s hypberolic, the dark matter is flung off into intergalactic space. If it’s elliptical, it orbits the merged galaxy like comets orbiting the Sun.
I will confess I am a little hazy on this point, as galactic formation isn’t exactly my area. But I am sure there is a way for the dark matter to settle down, because most of the galaxies we observe today are the result of mergers and have a nearly spherical dark matter halo. It may help to think of the conservation of momentum. The two clumps DM-A and DM-B have momentum in opposite directions, so some significant fraction of it will cancel out. Gravity alone is capable of this. To see why we have to undo some of the simplifying assumptions I’ve made and deal with angular momentum. The dark matter clumps are three dimensional objects, and parts of it are moving past other parts when they cross. That gives them some internal angular momentum. That in turn (see what I did there) will alter the trajectory, causing those pieces to “pull in” a little. Once that happens, they remain closer to the other chunks of dark matter than they would have done otherwise, and so the linear gravitational attraction is stronger. That pulls them towards-then-past each other faster, which gives them more angular and less linear momentum.
… Uch. That was not phrased elegantly. Short version: As DM-A and DM-B zip through each other, angular momentum condenses the ‘orbit’ until eventually it’s just a spinning ball of dark matter.