ASS out of U and ME

Assumption noun (BELIEF): something that you accept as true without question or proof
Cambridge Dictionay | Cambridge University Press

The gravity of the situation

Question: What is gravity?
Answer: We don't really know. We can define what it is as a field of influence, because we know how it operates in the universe. And some scientists think that it is made up of particles called gravitons which travel at the speed of light. However, if we are to be honest, we do not know what gravity "is" in any fundamental way - we only know how it behaves.
StarChild Question | NASA

A dark matter of fact

Though there have been several other suspected, similarly booted black holes elsewhere, none has been confirmed so far. Astronomers think this object, detected by NASA's Hubble Space Telescope, is a very strong case. Weighing more than 1 billion suns, the rogue black hole is the most massive black hole ever detected to have been kicked out of its central home ...
Researchers estimate that it took the equivalent energy of 100 million supernovas exploding simultaneously to jettison the black hole.

The most plausible explanation for this propulsive energy is that the monster object was given a kick by gravitational waves unleashed by the merger of two hefty black holes at the center of the host galaxy.
Gravitational wave kicks monster black hole out of galactic core | Phys org

ASS out of U and ME A small but distinctive signal in X-rays from the Milky Way could be key to proving the existence of dark matter. That is the claim of US scientists who analysed the energy spectrum of X-rays gathered by Nasa's Chandra satellite.
They found more X-ray photons with a particular energy than would be expected if they were produced only by familiar processes. Those photons could in fact have been generated by the decay of dark matter particles, say the researchers ...
"This result is very exciting," said Dr Abazajian, who was not involved in the research. "It makes it more likely that the line is due to dark matter." Scientists reckon that dark matter makes up more than 80% of all the mass in the Universe. As its name suggests, it gives off no light, but reveals its presence through the gravitational tug it exerts on stars within galaxies. However, we still have little idea about what dark matter actually is ...
Like other galaxies, the Milky Way is thought to be enveloped in a bubble of dark matter ... As expected, they found the signal in the latter to be stronger, given that dark matter should be densest where there are more stars - in the galactic centre.
The researchers also ruled out a couple of alternative astrophysical sources for the signal: photons emitted either when very large black holes suck in material from their surroundings or when ions of sulphur take electrons from hydrogen in the centre of galaxy clusters.

"We found that our result is consistent with previous results if you assume the cause to be dark matter," said Dr Bulbul.
Galactic X-rays could point to dark matter proof | BBC

The gravity of the dark situation

assume you make an ASS out of U and ME The arms of spiral galaxies rotate around the galactic centre. The luminous mass density of a spiral galaxy decreases as one goes from the centre to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it (similar to the solar system). From Kepler's Second Law, we expect that the rotation velocities will decrease with distance from the centre. This is not observed. Instead, the galaxy rotation curve remains flat as distant from the centre as the data is available.

If we assume the validity of Kepler's laws, then the obvious way to resolve this discrepancy is to conclude that the mass distribution in spiral galaxies are not similar to that of the solar system. In particular, there is a lot of non-luminous matter in the outskirts of the galaxy ("dark matter").
Galaxy rotation curves - Dark Matter | Wikipedia

Stars in bound systems must obey the virial theorem. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies[50] do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.

As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.
Velocity dispersions - Dark Matter | Wikipedia

Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways:

  • From the scatter in radial velocities of the galaxies within clusters
  • From X-rays emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
  • Gravitational lensing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).

Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.
Galaxy clusters - Dark Matter | Wikipedia