Astronomers don’t know what Dark Matter is. Dark Matter is a theory. No one has seen it, nor detected it in any way, shape, or form. This is important because no one knows what it is. But what they do know is that a huge amount of matter is required that currently is “missing”. How do they know it’s missing? Why is this material required? Primarily to provide the gravity needed to make our “models” of the universe reflect what we observe. To understand the concept of Dark Matter means we need to understand why it is needed.
Dark Matter is needed to make Newton’s law of gravity work on a galactic scale. Poor Newton….he just saw an apple drop from a tree and made a few laws of motion. Now, hundreds of years later we are asking him to extend his apple law to the motion of the Galaxies. I don’t think he ever saw this coming……
Ok, what’s the problem? There are two gravity related issues here, both involving galaxies:
The first is the gas cloud around our Galaxy, the Milky Way….it doesn’t fly off into space. This gas cloud is not visible but can be detected by x-rays, so it is there. It rotates around the galaxy at a substantially higher speed than predicted by Newton. It should, by all reason, fly off into space. …but it doesn’t. The only thing that we can think of is that there must be more mass between the edges of our galaxy and the gas cloud. That would keep the Gas cloud circling around the galaxy. For our galaxy, we pretty much know what is here, or can give a good approximation, so the extra mass that is needed to satisfy Newton must be invisible to us. That’s pretty much it…enter Dark Matter.
And the second issue is that Galaxy clusters don’t fly apart! A galaxy cluster consists of a hundred or so galaxies which are directly related by belonging to, and evolving from, a common hot hydrogen gas cloud, about 50 million degrees or higher! This hydrogen cloud contains several times the mass of the galaxies themselves. It is a big gas cloud! Nevertheless, given the velocities of the galaxies around the center of the cluster, the galaxies should be flying off in all directions….but they don’t! In order to explain this, we need more mass! Again….enter Dark Matter.
The quantity of Dark Matter needed to make the Newtonian Laws of Motion work for the above two issues is a lot of mass! …like 6 to10 times the mass of all the material in the galaxy, or in the galaxy cluster. That is a huge amount of mass! And this mass is invisible and undetectable! But physicists have some ideas about what particles might fill the bill and they are busy trying to find them. The story will continue, but for us common folk, when we read an article about Dark Matter, just remind yourself that the physicists are trying to figure out why galaxies and galaxy clusters are not flying apart….
There is also a minority view that does not require Dark Matter. This view suggests that there may be many more Ultra massive Black Holes hiding within the Galaxies and Galaxy Clusters so that sufficient mass is present to satisfy the Newtonian Laws of Motion. This could be possible since Black Holes contain millions, if not billions, of sun-masses in a relatively small volume….less than half the size of our solar system. This view however will require much more knowledge of Black Holes and their frequency of occurrence.
(Of course, the issue could be solved by letting Newton off the hook….maybe his Laws of motion work well here on earth and in our solar system, but on a galactic scale, i.e., millions of light years, gravity might just “march to the beat of a different drummer”.)