There is something wrong with our galaxy. What we know of physics, notably the theory of general relatively published by one Albert Einstein, tells us that the galaxies in the sky should have scattered their stars across the Universe long ago. There has to be an explanation why stars are held in galaxies. So let’s explore the concept.

To begin we should take a few steps back. We need to talk about gravity. Gravity is the unseen but ever felt force that holds us to our little blue and green planet. If there is something, there is gravity. If there is gravity, there is something. This applies to physical objects, radiation, and even time.

Now let’s narrow our focus from the galaxy scale to the stellar scale. Our Sun has a lot of gravity compared to our planet. This is because the sun has a lot more mass than our planet. It is so much greater that the sun hoards approximately 99.5% of the mass in our solar system. Because the gravity is so much stronger with the sun than our Earth, we are forced to obey the force of the sun’s gravity. The sun constantly tugs us toward it.

Right about now you should be shaking your head saying, “But Mike, if we are being pulled into the sun by gravity why haven’t we crashed into it already?” The answer is a mighty simple one. It is called centrifugal force.

Centrifugal force is not actually a force in and of itself, but a byproduct of momentum. It is just the tendency for an object traveling in a circle to push outward. This outward push is dictated by the mass and speed of the object. The faster and heavier the object, the greater the centrifugal force.

So the outward push of centrifugal force is working against the inward tug of gravity. This gravity tug on the orbiting planets is called centripetal force. The centrifugal and centripetal forces form a near equilibrium, with a slight advantage to the centrifugal push of orbiting. So our Earth is ever so slightly being pushed away from the sun, not toward it.

Although we are under the majority rule of the sun, that doesn’t mean that we have no effect. The gravity of the Earth, and of all the planets, has effect. This is made obvious by the fact that we don’t float off of the surface of the Earth during the daytime and crushed against the combined gravity of the sun and Earth during the nighttime. The Earth’s gravitational pull does affect the sun, if only in a minor way.

Now let’s apply those concepts to the galactic scale. The Milky Way galaxy we live in is analogous to our solar system. In our solar system we orbit around a star, in the Milky Way everything orbits around a hypothetical super massive black hole. In place of our planets we have stars which have low mass by comparison to the black hole in the center of the galaxy.

The super massive black hole is an extremely large black hole. Its physical size is believed to be slightly less than one astronomical unit (AU). An AU is the roughly the distance between Earth and the sun, or 150,000 kilometers, or just over 93,000 miles wide. I know what you are thinking. You are thinking, “That isn’t that big.” You are right. There are stars that are physically larger than this black hole. Just keep in mind that we are talking about mass, not size. Something can have more mass than another object which is physically larger. A bowling ball is heavier than a pillow but the pillow is larger. That bowling ball has more mass. This super massive black hole might be small but it has the mass of a million suns (solar masses) or more. This super dense, super massive, super awesome black hole is the center pulling force of the Milky Way galaxy.

So we have the super massive black hole in the center, and the stars orbiting it.

Now we come to our problem. Remember back a few paragraphs when I said that the Earth was slowly being spun away from the sun? Well that should be happening to the stars in the galaxy too. The combined gravity between all the stars and the super massive black hole in the center is simply not enough to keep the galaxy from spinning apart. Although our plant would spin away from the sun at such a slow rate that it would have ended its life long before, the stars in the galaxy have such a long life that they all would have diffused into the Universe long ago.

So what is keeping the Milky Way and other galaxies clustered together? By far the most accepted theory is that of Dark Matter.

So what is dark matter? If I told you I knew I would be lying. The fact is that my ignorance on the subject is shared with the world’s greatest astrophysicists. Dark matter theory is an invisible wild card in the scientific world.

Although no one can tell you “what” dark matter is it is possible to tell you “what it does.” Dark matter is an invisible substance that fills all space. It may be invisible but it is not magically invisible. Keep in mind that our ability to see things is determined by radiation reflecting off of objects, be it visible light or otherwise. If this dark matter did not reflect radiation or simply allowed light to pass through it, it would be invisible.

Dark matter is everywhere. If you were to cup your hands together you would be holding what? I would bet there is mainly , nitrogen, O2 ,and CO2 in your hands, but there is also dark matter. Not very much, but it is there.

Dark matter is also not very dense. This explains how we can walk around without bumping into invisible dark matter.Its density is so low that it doesn’t form enough gravity in our entire solar system to even be noticed.

Now let’s take into account the idea that the entire galaxy is full of dark matter occupying every nook and cranny. Even though all the dark matter in our solar system is insignificant, adding all the dark matter in the Milky Way up into the gravity pot of our galaxy creates a significant boost the Milky Way needs to not fly apart.

I am somewhat skeptical of the dark matter concept myself. The theory does not need to provide answers as to what dark matter is made of, but it would still be nice if we knew. Since we can’t see or detect it, we cannot disprove it. I can’t disprove Russell’s Teapot either, that doesn’t mean it exists. However for the moment the dark matter theory provides the best answer so I will accept it until something better comes along.

Images courtesy of NASA.