In early August, astronomers announced that they had created a map of dark matter — the mysterious and invisible matter that astronomers say underlies all structures in the universe — linked to some of the oldest galaxies in the universe.
Articles reporting on the achievement described an innovative observing technique: searching for tiny distortions of patterns in the cosmic microwave background radiation, the background light of the universe that originates from the Big Bang. These distortions appear because the mass bends space, even if that mass belongs to an invisible type of matter.
However, these reports haven’t delved into the mystery of what dark matter is, or question whether it actually exists. For most astronomers, most of the time, the fundamental nature of dark matter is completely off topic.
Dark matter, whatever it is made of, is important in our universe. Studying their distribution helps us understand how galaxies form and helps us discern the entire structure of the universe. But are we kidding ourselves here? Isn’t anyone bothered by the fact that we can’t see it and don’t know what it is?
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Although never directly detected, scientists actually have very good reasons to believe that dark matter is real. The first story everyone tells is about how galaxies spin at impossible speeds.
The stars on the outer edges of spiral galaxies are spinning around the center so fast that if nothing provides more gravity To hold them, they had already escaped into intergalactic space, like children being tossed out of a whirlpool spinning at great speed.
Proposed solution: invisible and intangible matter, supposedly made up of a collection of particles that we have all missed our terrestrial experiences, surrounds the misbehaving galaxy and penetrates it, providing its extra gravitational mass that observations require. Every galaxy (with a few possible rare exceptions) is embedded in a roughly spherical mass of dark matter we call a “halo”.
It is not unreasonable to suggest another possibility: we may not need something new to produce more gravity; Perhaps gravity works differently than we thought. This has been the main approach of dark matter skeptics in astrophysics, and when it comes to galactic rotations, it appears to be an attractive solution.
These modified gravitational models work so well to solve the spin problem that news articles appear in newspapers and magazines that regularly announce that dark matter has been refuted by a simple modification of Newton’s laws (or Einstein’s laws).
But there’s a reason we haven’t all gotten rid of dark matter and embraced the demise of gravity as we know it: the best evidence for dark matter comes from cosmic phenomena that occur on much larger scales than any galaxy, where there are fewer observing multiples and where the agreement with theory is Incredibly accurate.
The plethora of evidence would be compelling even if we ignore the rotation of galaxies entirely, and we still have to tweak the theory of gravity that can rival dark matter when it comes to everything else: the shapes of galaxies, galaxy cluster motions, gravitational lenses, and the abundance of elements. From the early universe, the distribution of galaxies on larger scales, and even patterns in light of the cosmic microwave background itself.
Even accepting that the astrophysical evidence is strong, it is understandable to remain uncomfortable with the idea of adding a new particle to the zoo of discovered species without any concrete detection of the particle itself.
Some of the simplest theoretical possibilities for the properties of dark matter particles have already been ruled out. But instead of giving up entirely, astronomers and physicists are constantly looking for new and creative ideas for what dark matter is and why it has yet to emerge.
Although not experimental, when all the evidence is taken into account, the idea that the universe has been completely overrun by invisible particles fits the data better.
There is a saying usually attributed to the statistician George Box, that “All models are wrong, but some are useful.” In cosmology, we sometimes openly describe our job as “solving the mysteries of the universe” but from a daily perspective, our job is to build and test mathematical models to describe the data we collect.
The lack of detection of the particle in the detector may make us uncomfortable, but it does not cancel any of the ways in which we see the influence of dark matter in the universe. And there’s no indication that dark matter should be something that interacts with detectors at all.
Some other solutions can still be found. But whatever it is, it should look, by observation, just like the set of invisible, untouchable particles that make up most of the matter in the universe.
Those of us who spend our time exploring the exciting boundary layer between particle physics and cosmology will continue to try to figure out what these things really are, while astronomers searching for new astrophysical data can take advantage of what we know about their abundance and behavior in order to try to solve other cosmic mysteries.
Whatever the dark matter is, we can be grateful for its role in holding all this ordinary matter together, and rest assured that it will likely continue to do a great job of preventing our Sun from zipping through the void.
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