The Bullet Cluster Proves Dark Matter Exists, But Not For The Reason Most Physicists Think
“When your cluster is undisturbed, the gravitational effects are located where the matter is distributed. It’s only after a collision or interaction has taken place that we see what appears to be a non-local effect. This indicates that something happens during the collision process to separate normal matter from where we see the gravitational effects. Adding dark matter makes this work, but non-local gravity would make differing before-and-after predictions that can’t both match up, simultaneously, with what we observe.
Interestingly, this argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes. This is a fundamental issue that must be addressed, if alternatives to dark matter are to be taken seriously as complete theories, rather than ideas in their infancy. That time is not yet at hand.”
Recently, a paper came out challenging alternative theories to dark matter and claiming that many of them were invalid. The basis for that argument? That those theories predict different arrival times for gravitational waves and light waves from a neutron star merger, when we saw them arrive practically simultaneously. One of those theories, MOG, claims to survive, but it’s already been discredited for another reason that’s discussed far less frequently: the Bullet Cluster. When the apparent effects of gravitation are well-separated in space from where we see the matter, you require non-locality to save your theory. MOG is a non-local theory of gravity, so you might think everything is fine. But if gravitational effects aren’t where the matter is located, we’d expect to see these non-local effects in clusters that are in a pre-merger state, and those don’t exist.