Big Bang theory is not valid.

[Anomalocaris]

If we don't know what dark matter 'is', then how do we know it's not baryonic???

You don't know who most of the contributors here are, yet you know they most have some command of English, right?

We do not know what Baryonic matter is. But we do know a little bit about what property it must have, and what properties it can not have. It so happens the little bid we know about what properties it can not have also happen to be some of the same properties which baryonic matter must have. So we know it's not baryonic matter.

Because we know pretty well what properties baryonic matter has, and how it interacts with each other and with well know nuclear processes, Therefore we can deduce exactly what effects large amount of baryonic matter must have on many observed processes in the universe. We don not observe those effects, therefore we can deduce there is not a large surplus of unobserved baryonic matter in the observable universe.

However, we do observe effects throughout the universe which best fit a large surplus of mass in the universe. Therefore we are fairly convinced there is a large surplus of mass in the observable universe that consist mostly of non-baryonic matter.

This should give you some idea of how science can reach out with rigor to say with well founded conviction about far more than just what can be seen.

But as a matter of fact, you can even sort of directly observe why most of dark matter is not baryonic. We know galaxies consist of high percentage of hidden mass that can not yet be directly observed. We know this because we observe the effect of the gravity of the unseen matter on the stars which we can clearly observe.

However, from our very well tested understanding of gravity, as our rigorous observation of motions of stars in galaxies, we can not only deduce the total amount of unseen mass in a galaxy, we can also deduce its distribution.

As it turns out, the unseen matter is distributed quite differently in a galaxy compared to visible baryonic matter. Calculation shows that in order for baryonic matter to be distributed in the manner we found that unseen matters are distributed, it must be uniformly very hot. Hot baryonic matter would be visible. Unseen matter is not visible.

So the unseen matter, because it is neither hot and visible, nor distributed like cool and invisible baryonic matter ought, is probably not baryonic.