However, a minority of scientists reject the dark matter hypothesis as implausible. Instead, they believe that the accepted laws of physics are incorrect. According to them, either the laws governing the motion of astronomical objects are wrong, or our theory of gravity doesn’t work on galactic scales. For both conjectures, these researchers have developed an array of new physics theories, governed by different equations than those taught in physics classes.
Both camps — the dark matter proponents and the modified physics community — point to different sets of astronomical data to support their position. And both groups can point to observations that support their conjectures and disfavor the other. While most astronomers embrace the idea of dark matter, there has been one observation that is extremely difficult for the dark matter camp to explain: the distribution of small galaxies surrounding bigger ones.
These smaller galaxies are called “satellite galaxies.” The two explanations — dark matter and modified physics — make different predictions about how satellite galaxies should be arrayed around galaxies like the Milky Way. For the past half century or so, astronomers have known that the observations favor the modified physics camp.
The Milky Way is a spiral galaxy, which means it looks a little like a spinning disk, about 100,000 light-years across and 12,000 light-years thick — essentially a cosmic pizza pan. This is the shape of the visible stars and galaxies. However, dark matter theory says that dark matter is essentially a big, spherical cloud, maybe 700,000 light-years across, with the Milky Way located at the center. Because dark matter is important in galaxy formation, dark matter theory suggests that the satellite galaxies of the Milky Way should also be spherically distributed around it.
On the other hand, if dark matter isn’t real, and the correct explanation for speedily rotating galaxies is that the laws of physics must be modified, scientists predict that the satellite galaxies should orbit the Milky Way in roughly the same plane as the Milky Way — essentially extensions of the Milky Way itself. When astronomers measure the location of the 11 known satellite galaxies of the Milky Way, they find that they are located in the plane of the Milky Way. Furthermore, the observed configuration is very improbable from a dark matter point of view. So, this is a win for the modified-gravity crowd.
Another win for dark matter
In other words, when the over-emphasis on the Leo satellite galaxies caused by the algorithm is taken into account, along with their temporary alignment in the galactic plane, the observations of the Milky Way’s satellite galaxies are now totally consistent with the dark matter hypothesis.
Granted, a single measurement is not enough to definitively decide the debate. However, it appears that one of the strongest examples of data favoring modified physics and disfavoring dark matter no longer has the impact it once had. Given the broad support by other data for dark matter, this paper has strengthened the case for it.