Then, to make matters much worse, along comes dark energy: An even more mysterious force, which we also cannot directly detect, that is effecting the rate at which the universe itself expands.
Above: The odd way galaxies spin, and why dark matter is proposed to explain it, all presented at a high school maths level - thank you TheCosmicWeb
Scientists really don't like having to explain the things they see in the universe by resorting to invisible matter, undetectable energy, or anything else unseeable. It's just too easy, as you can use it to explain almost anything. And the failure - to date - of our most sensitive instruments, including the alpha magnetic spectrometer mounted on the hull of the ISS, to find any definitive evidence of dark matter (beyond its gravitational effects which is what it was was proposed to explain) has led to some interesting non-dark matter explanations for the strange way we see galaxies spin, and the way the expansion of the universe seems to be accelerating.
|Above: The Alpha Magnetic Spectrometer, a tool for hunting dark matter, mounted onto the side of the ISS. Courtesy of NASA|
Mostly these explanations fall under the umbrella term of MOND (MOdified Newtonian Dynamics) a collection of theories that suggest that gravity doesn't work quite the way we think it does long distances. By and large they haven't managed to usurp dark matter and dark energy as the leading explanations for the strange observations, but they do represent some interesting thinking.
Above Neil deGrasse Tyson explains the MOND vs dark matter debate in a radio interview. Courtesy of Startalk radio
Today, however, I've run across a deceptively simple alternative to both dark matter and MOND. The idea being tentatively proposed, by P. Magain and C. Hauret from the Institut d’Astrophysique et de G ́eophysique in Belgium, is that maybe it isn't matter or gravity that isn't behaving like we expect.
Maybe it's time.
Einstien's theories of general and special relativity tell us that it's possible for the flow of time toi be changed locally by either an object moving very fast, or an object having a very powerful gravitational field. But on the largest scales cosmologists usually assume that the flow of time is pretty much even. Magain and Heurets suggestion - which fits some observations almost perfectly but has yet to be tested against others - is that the flow of time on very large, universal, scales is tied to the entropy of the universe, and so might vary. If time can vary as we look out across the universe, then that might explain some of the strangely moving things we see - they are moving according to the physical laws we know, no dark matter or energy needed, but we are seeing them out of sync with each other..
I'm only going to put up this quick note on the subject at the moment, but the simplicity and audacity of the idea makes me wonder if it doesn't have alot of potential, so watch this space. The paper is here