# Tag Info

### 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, ...
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### 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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### 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 - ...
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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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### 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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### 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 ...
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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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### Could light be dark matter?

Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain. The force ...
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### 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 ...
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### 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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### 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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### 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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### 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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### 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 be ...
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### 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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### What are the differences between matter, dark matter and antimatter?

Matter is the stuff you are made of. Antimatter is the same as matter in every way, looks the same, behaves the same, except its particles have electrical charges opposite to matter. E.g., our ...
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### 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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### 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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### 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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### 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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### Why aren't brown dwarfs the dark matter?

Two reasons. We know from looking at galaxy rotation curves and the motion of galaxies in clusters and from gravitational lensing, that the amount of "dark matter" is some 30% of the density of the ...
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### 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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### 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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### 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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### Is there a strong galactic magnetic field?

No, the galactic magnetic field is very weak, about 0.1nT. It is able to bend the trajectory of highly-energetic charged particles and also to align dust grains across the magnetic field. However, is ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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The paper describing this finding is van Dokkum et al. (2018). In a galaxy, the ratio of the stellar mass $M_\star$ to dark matter mass $M_\mathrm{DM}$ is normally very small, increasing with mass ...