DARK MATTER

1.DARK MATTER IS NOT SIMPLY NORMAL MATTER THAT WE CANNOT DETECT.

This is something that's completely known. Dark matter cannot be: failed stars, clouds of gas, or dust grains, asteroids or comets, basketball-sized clumps of normal matter, an ionized plasma, black holes, or anything else made originally from normal matter. We have a suite of evidence that rules out that possibility. The dark matter structures which form in the Universe (left) and the visible galactic structures that result (right) are shown from top-down in a cold, warm, and hot dark matter Universe. From the observations we have, at least 98%+ of the dark matter must be either cold or warm; hot is ruled out.

2.Dark matter must not interact very much with itself, with light, or with normal matter.

There's no doubt that if dark matter exists, there must have been a pathway for its creation in the young Universe. However, whatever that pathway was, those interactions are no longer occurring and haven't occurred with great abundances in a very long time. Direct detection experiments haven't revealed dark matter, constraining its possible mass and cross-section. It doesn't absorb or blur distant starlight, restricting its interactions with light. It doesn't annihilate with itself above a certain threshold, otherwise a large and diffuse gamma-ray signal would be seen at the centers of galaxies.  

In fact, it's 100% consistent with not interacting at all via any of these mechanisms. Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. If new research is correct, then dark matter will obey a different distribution depending on how star formation, over the galaxy's history, has heated it. The dark matter-to-normal matter ratio of over 600-to-1 is the greatest ratio ever seen in the dark matter-favoring direction.

3.Dark matter causes gravitational effects in places where normal matter isn't located.

This is some of the strongest evidence of all that dark matter cannot simply be normal matter that's dark. When two galaxy groups or clusters collide, the intergalactic gas and plasma collides and heats up, emitting X-rays (shown in pink). This represents the overwhelming majority of the normal matter, far more than what's found in stars and the individual galaxies themselves.

However, just because there are things we know about dark matter doesn't mean that we know it all. In fact, here are five major things we don't know about it. 1.) We don't know what particles are responsible for dark matter, or if it's even a particle at all. 2.) We don't know whether the "dark sector" is simple or rich. 3.) Did dark matter always exist in the Universe, or was it created at some later time? 4.) Is dark matter eternally stable, or will it all someday decay away? 5.) Will any of our direct detection experiments ever find it, or is this a fruitless endeavor?

What do we know about dark matter?

Dark matter is the stuff that makes it possible for galaxies to exist. The gravity of the visible matter is strong enough to form galaxies and complex structures. The stars would more likely to be scattered all over the place and not form galaxies. So, we know there is something else inside and around them. Places with a high concentration of dark matter light near by. So we know there is something there that interacts with gravity.