Recently a team of astronomers claimed to have found evidence for ancient massive stars built from Dark Matter. It's a pretty cool - speculative – result. While I could do a post just on that topic, I thought you might want a simpler, more straightforward question answered first.
What the hell is Dark Matter anyway?
Remarkably, nobody knows the answer to this question . Even more remarkable is the very real possibility that Dark Matter doesn’t exist at all. The problem Dark Matter seeks to solve is, however, very real and very pressing for astrophysics. That’s what makes the whole story so cosmically fascinating.
Let’s start with the basic idea. You, me, and everything we can see in the Universe is made up of what physicists call “normal matter.” You can see normal matter because it emits light. Dark Matter, on the other hand, is supposed to be a form of stuff that doesn’t emit light and only interacts with normal matter via gravity. According to our best estimates about 85% of all the stuff in the Universe is in this dark form. In other words, Dark Matter rules.
Unless it doesn’t because it’s a fiction.
So, where did the idea of Dark Matter originate? It starts with Newton’s law of gravity which tells astronomers exactly how much gravitational force massive objects will exert on each other. The attractive gravitational force provides astronomers with a direct link between mass and motion. More mass, more gravitational attraction, faster motions.
In the 1970’s, astronomers started using Newton’s laws in this way to measure the mass of galaxies. They looked at how fast stars in a galaxy were moving around the galactic center. Then they used their measurements to “weigh” the total mass in the system. What they found was the galactic mass - measured via the gravitational motions - was much larger than what they found from counting stars and other forms of visible matter. Soon, many other examples of stuff moving too fast and, therefore, weighing too much were found in the cosmos.
This led to a come-to-Jesus moment for astronomers. Something was clearly not working. Either Newton’s law of gravity was wrong or there was a huge amount of invisible stuff out there exerting gravitational forces on the visible stuff we could see. Most of the astrophysical community went all in on the “huge amount of invisible stuff” idea. That’s how we ended up with Dark Matter.
Now here is the kicker. It’s 50 years on and we still have no idea what Dark Matter is. All normal matter is made of particles like electrons, protons, etc. So where are the Dark Matter particles? Decades of experiments have failed to find anything.
This is a serious big-ass cosmic mystery. If we can’t figure it out then we don’t really have a handle on the Universe. There’s no doubt about the observations showing normal visible matter moving too fast. But is it really caused by some kind of invisible dark matter stuff or is the Universe telling us it is even weirder than that?
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PS. Is Dark Matter real or are we in for a surprise? Leave a comment on the website version of the post or email me at [email protected]
PSS: Dark Energy is something totally different from Dark Matter so we will take it up some other time.
— Adam Frank 🚀
