So lets start this from the beginning - starting with observations in the 1930s, and throughly confirmed in the 1970s, astronomers noticed that the motion of stars & dust clouds orbiting around the center of their galaxies was very odd. Stars in the outer portion were moving around the center much faster than they should; the visible matter not having anywhere near the mass needed to hold things together. This is called the galactic rotational curve problem. This led to two competing hypothesis; the first being that at large scales gravity functions differently than what we see in the solar system. The second is that there is a bunch of difficult to see (ie "dark") matter whose gravity is keeping stars from flying off.
The dark matter theory is currently in favor, due to a number of observations that make sense if invisible matter is a thing, but don't really fit the modified gravity theory. For example, there are areas like the Bullet Cluster where we see gravitational lensing that occurs in a way that does not line up with the visible matter. Or late last year a small number of dwarf galaxies were discovered that don't have the rotational curve problem; the orbits of their stars & dust clouds are what we would expect from the amount of visible matter. The dark matter hypothesis can explain those galaxies as 'simply' not having much dark matter for whatever reason, while they are a difficult fit for modified gravity theories.
>>12149307I wonder if anyone has mentioned those oddball dwarf galaxies to McCulloch, and asked him if QI has any explanation for them.
