Why isn't dark matter just matter ?
The evidence usually presented in support of dark matter is larger-than-expected masses in galaxy clusters: the mass required for the gravitational lensing is far larger than the mass detected in the galaxies themselves.
So why, then, is the assumption that this undetected extra mass is a previously-unknown kind of matter? Why is that new concept required, instead of proposing that the estimates of total normal matter are wrong?
Another way of asking the same question could be: Why are we more sure of our estimate of the sum of normal matter than of the evidence that there is more mass in galaxy clusters than estimated?