92

Great question. Observations show that Dark Matter (DM) only noticeably interacts gravitationally, although it's possible that it may interact in other ways "weakly" (e.g. in the 'WIMP' model --- linked). Everything following has no dependence on whether DM interacts purely/only gravitationally, or just predominantly gravitationally --- so I'll treat it as ...


56

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 the debris of a dark matter decay that occurred far from Earth), and it currently reports an excess of gamma rays. There are occasional claims that nontrivial ...


49

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 the money right now.2 And that is not unusual. The history of missing-mass/dark-matter is one of proposals being made and then ruled out one-by-one, in order ...


40

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, suppose we take the scale factor of the universe, $a(t)$, to be unity right now and we take the current average density to be $\rho_0$, then at a time $t$ the ...


38

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." When Ting sold NASA and DOE on the AMS, he said it might find runaway particles from oases of antimatter, helping solve a deep riddle. The big bang produced matter ...


29

The answer comes from the virial theorem, which can be derived from the Jeans equations, which are the equivalent of the Euler equations of fluid dynamics for collisionless particles (i.e., dark matter). Incidentally, the virial theorem is also valid for an ideal fluid. For a derivation see Mo, van den Bosch & White 2010 (or I'm sure many other texts). ...


27

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 we do not have any direct means of measuring it.


26

Dark matter would affect planetary motion, but the influence of dark matter on planets in our solar system is too small to detect even currently due to the low concentration of dark matter compared to ordinary matter in our solar system. See Constraints on Dark Matter in the Solar System. The density of dark matter is very low, $ <~10^{-19} grams/cm^3$,...


26

Why shouldn't the orbits of stars be Keplerian? The answer is simple. Keplerian orbits are predicated on a single central point mass. That assumption fails to some extent even in a solar system. It fails massively in a galaxy. A galaxy is not a point mass.


25

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 particles "get" mass from, as if mass is a resource like money and the Higgs field is giving it out. But that's not right. In a generic field theory there is no ...


24

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 Review D, the lower bound on the mean lifetime of dark matter particles is $3.57\times 10^{24}$ seconds. This is roughly $10^{17}$ years. By comparison the age of ...


24

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 something like: $$ \rho \propto \frac{1}{V} \propto \frac{1}{a^3} $$ where $a$ is the linear scale factor (i.e. $a^3$ is the volume scale factor). However things ...


23

We don't know. Though there are several ideas what dark matter could be, e.g. the humorously abbreviated WIMPs, all we know about dark matter is that is it massive (by light deflection, etc., etc.) and that it does not interact electromagnetically, and probably also not with the strong force. Other than that, there is no sufficently tested theory of dark ...


23

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 different energies of the proposed dark matter particles. This temperature (=kinetic energy of the particles) is an important factor for the dark matter behaviour ...


22

As a general rule, zero mass particles which travel with the velocity of light are not good for dark matter, because dark matter concentrates around gravitational attractors. It has to be particles with some mass that can be at rest in order to stay around a galactic center from the beginning. In addition they have to be controlled by weak interactions, if ...


22

I think the problem with matter that only interacts gravitationally is that it's hard to get it all to stay in one place. Nebula slowly form stars and planets in part because of collisions between particles lead to larger particles, which tend to attract further particles. But particles that just wizz right through each-other can't coalesce without violating ...


22

The title and the text actually ask two different questions. While Kyle Oman and Thriveth answer the title excellently, I'll address the question in the text which asks "Why did the Universe expand in the first place, before dark energy (DE) started to dominate". The answer to this is inflation (we think). The first fraction of a second after the creation ...


22

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. In this picture, created in 2003, two galaxy clusters have collided and passed through each other. The red coloring indicates where X-rays from high-...


21

The show you watched seems to get two concepts mixed up: Supersymmetry and Dark Matter. The existence of Dark Matter is strongly hinted at by comsological and astrophysical considerations. It is the easiest explanation for several observations we make in the universe. Supersymmetry on the other hand provides a candidate particle. The lightest ...


21

Let's start partway through the expansion of the Universe in the matter dominated epoch. At this time the energy density is dominated by matter, but the dark energy and radiation components are still present, just relatively small. The Universe is expanding, but the expansion is gradually slowing down. As the Universe expands, the density of matter scales ...


20

Because the dark matter does not interact a lot, there is no mechanism that would slow it down quickly. When a dark matter particle is falling towards some gravitational center, it is speeding up, then it flies through the periapsis and continues away into the distance. Normal matter clumps into planets, because it is slowed down by interactions / collisions....


20

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, electromagnetic radiation. "Normal" matter is able to dissipate kinetic energy and as a result can fall deeper into a potential well. Yes, dark matter is ...


20

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 potential and then shoot out the other side. To be clear - this answer assumes that the "non-ordinary" dark matter that the question refers to is non-baryonic ...


20

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. The rotation curves she observed were flat ($\color{green}{\mathrm{green}}$ in the diagram below), showing that stars revolve much faster than expected based ...


20

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 uncertainty principle. The electromagnetic field is just one of many quantum fields in the universe, and there's no law saying you have to interact with it. Kaku ...


19

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 particle would have an annihilation cross section of about $10^{-26} \text{ cm}^3/\text{s}$. Very general thermodynamic principles predict that thermal production of ...


19

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 dark matter is the rotation curve of galaxies. We found that galaxies don't rotate like the luminous matter suggests it should rotate. Specifically, given some ...


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