57
votes
Accepted
Is there any evidence that dark matter interacts with ordinary matter non-gravitationally?
There are some standing anomalies that could be explained by non-gravitational dark matter interactions. For example, Fermi-LAT is an indirect detection experiment (i.e. an experiment that looks for ...
- 105k
54
votes
Accepted
Why do physicists assume that dark matter is weakly interacting?
The short answer is that they don't assume that.
But among all the proposals that remain for what dark matter might be, weakly interacting stuff is the easiest to detect,1 so that is what is getting ...
39
votes
Is Dark Matter called "Matter" only because of gravity?
As the universe expands the density of matter goes down. For example if the volume of some specific region of the universe doubles then the density of the matter in that region halves. More precisely, ...
- 363k
39
votes
Accepted
Why is Anti-helium so important in the search for dark matter?
The original experiment was designed to find it as a proof of antimatter, not dark matter.
the AMS is finally delivering on the promise of its original name when "AM" stood for "antimatter."
...
- 235k
36
votes
Mass of All the Neutrinos
The neutrino mass, as you have defined it, will not be part of any calculation of the visible mass of the universe. There will indeed be a very large number of $>$MeV neutrinos whizzing about that ...
- 137k
27
votes
What is the temperature of dark matter?
No. Because it does not take part in the electromagnetic interaction, dark matter neither absorbs nor emits electromagnetic waves. This means that if dark matter particles do have a "temperature" then ...
- 97.8k
26
votes
Accepted
What gives mass to dark matter particles?
I think this question contains a misconception unfortunately caused by popular science descriptions of the Standard Model.
The question seems to assume there needs to be some concrete source that ...
- 105k
24
votes
How long has dark matter been around?
How long has dark matter been around?
Short Answer
Almost forever.
Long Answer
Assuming a dark matter particle paradigm, according to a pre-print by Yang (2015) subsequently published in Physical ...
- 3,967
23
votes
Accepted
When galaxies collide, what happens to dark vs. light matter?
This is exactly what occurred in the collision in a galaxy cluster known as the Bullet Cluster. The name comes from the red shock wave to the right of the picture, indicating a collision in the past.
...
- 24.7k
23
votes
Is the term 'matter' in 'dark matter' misleading?
As the universe expands you would expect the density of the stuff in it to decrease for the obvious reason that the amount of stuff remains constant while the volume increases. So you'd expect ...
- 363k
23
votes
Accepted
Why is dark energy dominant between galaxies but not inside galaxies?
These aspects of astronomy and cosmology are indeed very interesting and very significant, but don't allow the names to get in the way of your understanding. Dark matter is a form of matter made (most ...
- 61.8k
22
votes
Why do physicists assume that dark matter is weakly interacting?
It's not just the "look under the lamp post" effect. There's also the "WIMP miracle". A new heavy (i.e. about the mass of the top quark, the heaviest SM elementary particle) weakly interacting ...
- 49.4k
22
votes
What is the temperature of dark matter?
These are actually two questions.
The temperature of the dark matter is an important value of many cosmological models. There are "hot" dark matter, "warm" dark matter and so on, models, proposing ...
- 8,249
21
votes
Accepted
If our galaxy's dark matter halo is so large and diffuse, why is the ordinary matter in it so much more localised and compactly located?
To answer your two questions:
Almost by definition, dark matter does not interact with itself or other matter at all (or only very weakly). It therefore does not dissipate its energy as, for instance,...
- 137k
21
votes
Accepted
If dark matter can't lose kinetic energy, then why is it not traveling at relativistic speeds?
If dark matter consists of particles that thermally decoupled from the rest of the universe very early, then its momentum distribution with respect to the comoving reference frame was fixed at that ...
- 137k
21
votes
Attraction between dark matter
Dark matter does attract dark matter via gravitation. Now in normal matter, which is bulk neutral, when gravity bring it into contact with other matter (at speed), the atoms collide via ...
- 39.6k
20
votes
Accepted
Can the Sun / Earth have a dark matter core?
The easiest way for dark matter to become trapped inside another object is if it interacts and loses some kinetic energy. Otherwise it would just gain kinetic energy as it fell into a gravitational ...
- 137k
20
votes
How did we 'discover' dark matter?
Although it seems Fritz Zwicky can claim priority for having postulated dark matter (see freecharly's answer), Vera Ruben's observations of galaxies' rotation curves gave more evidence to the claim. ...
- 2,166
20
votes
Accepted
If perfect blackness violates the Uncertainty Principle, how isn't dark matter a violation?
There's no contradiction, but Kaku is being pretty cavalier.
In both classical and quantum physics, an uncharged point particle can never emit or absorb any light. Nothing about this violates the ...
- 105k
19
votes
Accepted
Could dark matter consist of the supermassive black holes at the centers of galaxies?
Do the black holes at the center of galaxies account for the experimental results that prompted the introduction of dark matter? No
The primary piece of evidence that originally sparked the idea of ...
- 4,119
18
votes
If perfect blackness violates the Uncertainty Principle, how isn't dark matter a violation?
Every charged object, when accelerated, will emit electromagnetic radiation. This means that as long as atoms are jostling about on the surface of a solid due to random thermal motion, they will emit ...
- 97.8k
18
votes
Accepted
What is the temperature of dark matter?
Dark matter might be in the form of thermal relics (e.g. WIMPs) or they may be produced non-thermally in phase transitions (e.g. axions). In the latter case, it does not make any sense to talk about a ...
- 137k
18
votes
Could Hawking radiation have a lot of neutrinos and dark matter?
Yes.
Hawking radiation is universal: the black hole is a modification of spacetime geometry, and the quantum fields, which manifest particles, are residents of that spacetime. Thus they are all ...
- 20.8k
18
votes
Is Dark Matter in Motion?
Yes, dark matter has to be in motion, otherwise it would fall in to the galactic center. From the fact that galaxies are stable, we can expect the Virial Theorem to hold, i.e. that dark matter has a ...
- 6,495
17
votes
Is Dark Matter called "Matter" only because of gravity?
Too long for a comment, but on the nomenclature, Fritz Zwicky's observations of other galaxies made it apparent to him that galaxies had to have much more mass than could be seen. The visible stars ...
- 5,718
17
votes
Accepted
Why is the distribution of dark matter in a Galaxy different from the distribution of normal matter?
In the case of conventional matter, particles, atoms and molecules of it can interact electromagnetically and share energy, and they can do so in response to the effects of gravity. For example, in a (...
- 97.8k
17
votes
If dark matter can't lose kinetic energy, then why is it not traveling at relativistic speeds?
Now that's an interesting question. And truth be told, as with so many things about the characteristics of dark matter, we do not know the answer yet.
First, it is not correct that "dark matter ...
- 6,495
16
votes
Are modified theories of gravity credible?
Yes. Modified gravity theories are credible. Daniel Grumiller and Jerry Schirmer have pointed out some of the case against it, but there are deep, potentially intractable problems with a dark matter ...
- 3,967
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