34
votes
Why doesn't dark matter clump strongly in the center of galaxies, since it doesn't feel either radiation pressure or the Pauli exclusion effect?
The reason is the fact that dark matter is non collisional. The dark matter particles interact only gravitationally, they feel no pressure, right, but they also feel no drag! No drag, no friction, ...
- 4,371
31
votes
Accepted
Can there be planets, stars and galaxies made of dark matter or antimatter?
Dark matter galaxies are possible but very speculative. On a theoretical level, they are hard to form because dark matter interacts only gravitationally (see Anders Sandberg's answer), which makes it ...
- 3,992
28
votes
Can there be planets, stars and galaxies made of dark matter or antimatter?
Probably not. Dark matter should really be called "transparent matter" since it does not interact with light. This has an important consequence: it is hard for dark matter - whatever it is - ...
- 14k
28
votes
Accepted
How would "dark matter", subject only to gravity, behave?
What you describe is the standard paradigm in cosmological physics, so it has been studied extensively. The basic consequence of dark matter not having significant nongravitational interactions is ...
- 4,260
27
votes
Accepted
Does the recent news of "ten times more galaxies" imply that there is correspondingly less dark matter?
All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has ...
- 146k
26
votes
Accepted
If dark matter bends light, how do we know the stuff in the sky is where we think it is?
The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ ...
- 36k
25
votes
Is Dark Matter possible if there is dynamical friction?
Dynamical friction is considered in the study of dark matter.
The timescale for dynamical friction to cause a significant loss of kinetic energy is roughly
$$\tau \sim \frac{10^{5}}{\sqrt3 \ln \Lambda}...
- 146k
24
votes
Accepted
Could there be dark matter black holes?
The problem with trying to form a black hole with dark matter is that dark matter can only weakly interact (if at all) with normal matter and itself, other than by gravity.
This poses a problem. To ...
- 146k
24
votes
Accepted
Could dark matter be wrong?
Multiple theories and hypotheses have been proposed as an alternative to dark matter (DM). The most popular are, arguably,
MOND (MOdified Newtonian Dynamics):A term for various theories where the ...
- 37k
19
votes
What is the estimated density of the dark matter?
The average density of dark matter over the entire universe is estimated to be $2.2 \times 10^{-27}$ kg/m$^3$ (sometimes quoted in the odd units of $1.2 \times 10^{-6}$ GeV/cm$^3$ or the more ...
- 146k
18
votes
Accepted
Can dark matter be found in the shape of planets, galaxies etc.?
Planets and stars, no. Globular clusters and galaxies, yes.
Small scales
To condense into such relatively compact objects as planets, stars, and even the more diffuse star-forming clouds, particles ...
- 37k
18
votes
How can dark matter be observed?
The purple is a "weak lensing" map. To quote from the original authors:
WEAK LENSING MASS RECONSTRUCTION OF THE INTERACTING CLUSTER 1E0657-558: DIRECT
EVIDENCE FOR THE EXISTENCE OF DARK ...
- 115k
16
votes
Accepted
Latest cosmological parameters
Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 ...
- 37k
16
votes
Accepted
Is normal matter always accompanied by dark matter and vice versa?
We lack the precision to say that there aren't regions where there is matter without dark matter or vice-versa. But what is clear is that the ratio of dark matter to normal matter, which is (or needs ...
- 146k
15
votes
Accepted
Scientific evidence against CMB originating from “Oort Soup”?
The idea that belts or spheres of dust might be responsible for (some) microwave emission is not crazy. Indeed we know that dust does emit microwaves and indeed, the contribution of such dust has to ...
- 146k
15
votes
Accepted
Is there more dark matter than we previously thought?
Probably nothing changes
Several reasons for this:
The study actually asserts (last paragraph of main text) that:
The IMF variation also calls for an extensive revision of star formation rates and ...
- 3,992
14
votes
Accepted
How can the (in my eyes quite ridiculous) conjecture of Sheldrake be disproven?
Firstly, thank you for your leveled and clear explanation of Sheldrake's essay. I agree with you that it is quite ridiculous to make such a bold claim when there is such little support for it even for ...
- 4,439
13
votes
Conventional matter to dark matter ratio, outside of galaxies
Let me see if I can answer at least some of this.
Yes, there is dark matter between galaxies. This is demonstrated by the fact that in galaxy groups and clusters, you need more dark matter than is ...
- 16.4k
13
votes
Accepted
Are black holes expected to contain the same ratio of dark matter to regular matter as the rest of the universe?
(Short answer: No, scroll to the last point.)
It is irrelevant to an external observer whether the matter that fell into the black hole was dark matter or baryonic, by the no hair theorem. The only ...
- 449
13
votes
Accepted
Why aren't neutron stars full of dark matter?
Yes, neutron stars might actually accumulate weakly interacting dark matter and this allows some observational constraints on its nature. Basically, the temperature and continued existence of neutron ...
- 14k
13
votes
How can the (in my eyes quite ridiculous) conjecture of Sheldrake be disproven?
To be fair, Sheldrake credits Greg Matloff (2015) for this "dark matter is really the motions of 'volitional stars'" idea.
It's easy enough to show this won't work (I mean, aside from all ...
- 16.4k
13
votes
Accepted
Could we detect dark matter by black holes gaining unexplained mass?
The accretion rate is far too small to make much difference to Galactic black holes, but how could this be distinguished from the accretion of normal, baryonic matter in any case?
In fact it is easier ...
- 146k
11
votes
Is Anti-Dark Matter a thing?
Yes, it is a thing.
Weakly Interacting Massive Particles (aka WIMPS) are thought to come in matter and anti-matter forms and have a self-annihilation cross-section in order to produce the "correct" ...
- 146k
11
votes
Accepted
Why "Dark matter" is still considered?
I'm going to answer this question, not because I believe you want a thoughtful, coherent answer, but rather for others who may come across this question and look at the propaganda you've posted. I don'...
- 14.9k
11
votes
Accepted
Could not slow neutrinos orbit galaxies and clusters, thus comprising a large component of even COLD dark matter?
These neutrinos would have to be really cold. The cosmic neutrino background is at 1.9K, and they are considered hot dark matter, because they would have been highly relativistic at the epoch of ...
- 146k
10
votes
Could there be dark matter black holes?
As pointed out by Rob Jeffries, forming a black hole (BH) from dark matter (DM) is impossible (unless there is a [hypothetical] interaction by which dark-matter can lose energy that evades all ...
- 5,516
10
votes
Accepted
Could undiscovered smaller black holes within galaxies be an explanation for dark matter?
MACHOs (massive compact halo objects) are thought to make up a portion of the dark mass and portion of these are thought to be black holes, but there are several very solid reasons why black holes not ...
- 1,855
10
votes
How is dark matter different from ordinary (visible) matter?
There is certainly one and possibly two types of dark matter to solve two dark matter problems.
The first dark matter problem is that most gravitating matter is dark. Most of the inferred mass that ...
- 146k
10
votes
Do the Bullet Cluster remnants prove that dark matter consists out of particles?
TLDR: Not as such, but it does give us some constraints on its properties.
Background:
A galaxy cluster consists of three major components, from smallest to largest proportion of mass: galaxies (...
- 2,275
10
votes
Accepted
Are black holes considered part of baryonic content of universe?
Black holes formed after the epoch of primordial nucleoynthesis would be part of the baryonic matter, since they would be mostly formed of baryons. It is possible or even probable that every black ...
- 146k
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