Question

how can we identify dark matter?

Answer 1

dark matterwill not contribute electromagnetic radiation in the detectable spectra,i.e. visible,x-ray and gammas in a strength to be seen in our detectors

Answer 2

Identifying Dark Matter can be the wrong question right now with respect to the amount of knowledge we have about it. How can we feel it's presence, should be the right one.

1.
It shows it's presence by making us think that how the stars in these galaxies are holding on even when the speed with which they are revolving is so high that they could have flew off the cluster. Well, if we think of gravity, it will be considered as an extremely weak force. So it can't be and this is where the idea of dark matter becomes appropriate. We need some kind of matter that must have high gravitational forces that could overcome the centripetal force from the revolving stars. And also, we can't see it as it doesn't emit light. So let the name be Dark. Hence, Dark Matter.

2.
It is also observed that the light passing by the galaxies bends around them in a strange manner. We know that other bodies like stars and planets can do the same, but in this case the deflection is far more. That means, a stronger force similar to the gravitational force is there (as it also interacts with light too). Then let it be Dark Matter again.