57
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Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?
Ultimately, the presence of invisible mass is the most straightforward explanation for what we're seeing. We're aware that we could be wrong, but if we are, we're wrong about some of the fundamentals ...
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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, ...
- 4,471
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 ...
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29
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How are we observing the newly discovered "dark galaxy" J0613+52, if it has no stars and is so far away from other galaxies?
The low surface brightness survey at the GBT is looking for H(I) emission, i.e. emission from neutral hydrogen atoms (for example see O'Neil 2023).
The most obvious signature they use is the 21 cm ...
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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 - ...
- 15k
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 ...
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27
votes
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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 ...
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26
votes
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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 $0.001$-$0.01M_{\odot}$ ...
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26
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Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?
Let me use an analogy as an example of being able to say something about a thing we never see.
I can fairly confidently say that Zinn exist. Why can I say that? I'...
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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}...
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24
votes
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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 ...
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20
votes
Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?
"Is there something wrong with the way I'm thinking about this?"
Fundamentally, yes, but it's a matter of the philosophy of science as much as the science itself.
The question makes the ...
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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 ...
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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 ...
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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 ...
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16
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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 ...
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16
votes
Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?
This is a slightly more technical answer that should nonetheless illustrate the concepts.
Wind back the clock to 1845. You are an astronomer of that period, and you are familiar with Newton's Laws, ...
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15
votes
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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 ...
- 156k
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 ...
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15
votes
Accepted
Is the amount of dark matter the same as you look back through time (further away from earth)?
Unclear, but the operational assumption is that the amount of dark matter does not change.
Modern cosmology is based on the Friedmann equations, for which the energy density of matter, $\Omega_m$, is ...
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15
votes
Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?
We've extensively studied dark matter properties using the effects of its gravity. It's no different than the Sun, really. We've only know what the Sun is made of and where it gets its energy from for ...
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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 ...
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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 ...
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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 ...
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13
votes
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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 ...
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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 ...
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13
votes
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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 ...
- 156k
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" ...
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11
votes
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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'...
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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 ...
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