Not very long ago, astronomers and astrophysicists realized something was wrong with the Universe. The Universe is expanding, but the speed of that expansion is increasing. It shouldn’t be. The gravitational pull of the Universe’s mass should be slowing it down, or at least it should be maintaining the same speed. For it to be speeding up, something has to be accelerating it.
In addition, the stars on the outer fringes of galaxies rotate around the galactic center at a much faster rate than they should, based on the amount of matter in the galaxy (according to both classical and modern physical theory).
To explain the first of these phenomena, physicists have postulated that some sort of energy suffuses the Universe but is currently undetectable, and this energy is pushing galaxies apart at an ever increasing rate. They call it “dark energy.” To explain the second phenomenon, they postulate that there is another form of matter that is also currently undetected. This so-called “dark matter” surrounds galaxies and provides the additional mass (and gravitational influence) required to explain the curious motion of the stars.
Physicists have come up with a variety of hypothetical particles that they think could comprise dark matter. Some of it could be hot (i.e., fast moving), but if all of it were hot it wouldn’t explain the observed motion of stars and galaxies. So most dark matter theorists think at least 95% of it is cold, i.e., slow moving. Of this cold dark matter, they feel the most likely candidate is a “weakly interacting massive particle,” or WIMP. These WIMPs would rarely or never interact with regular matter, or even with other WIMPs, but they would form clouds whose gravitational attraction would form stars into galaxies and affect the stars’ rotational velocities.
Other possible dark matter particles might be self-interacting via dark electromagnetism that allows them to exchange dark photons and form dark atoms. The dark electromagnetism would not affect “normal” matter, but the dark matter would form disc-like structures alongside the disc of normal matter in galaxies, and their gravitational influence would also affect the motion of normal matter.
Several experiments are underway to try and detect at least some dark matter particles, especially WIMPs, but so far these attempts have been unsuccessful.
In reading about all this, I was struck by the idea that maybe physicists were bending over backwards to come up with increasingly complex theories to explain their observations when a simpler one might be better. In a way, it reminded me of the way astronomers in the 16th Century developed epicycles to explain the motion of planets in order to adhere to the notion that the Earth was the center of the Universe. When Copernicus showed that the Earth revolved around the sun and not the other way around, the motion of the planets was explained much more simply and elegantly.
Some physicists have made this same comparison, though they reject it because the idea of dark matter explains observations so well. Well, so did epicycles, at the time.
I think there might be another explanation that does the job without inventing a whole menagerie of new but undetectable particles.
For some time now, physicists have entertained the idea that we might live in a multiverse. That is, in addition to the universe we inhabit and which we can observe, there might be other universes, perhaps even an infinite number of universes. What if one of those universes were right next to ours? What if both our universes occupied the same meta-space, with gravitational effects from each one leaking across the dimensional divide to affect the other?
Current astrophysical theory makes this a possibility, and it doesn’t require conjuring up what seems to be a whole menagerie of semi-mythical particles and energies.