[dventimi]

People and scientific instruments are made of "normal" (non-Dark) matter, which makes it easy to detect other "normal" matter (electrons, protons, quarks, photons, neutrinos, what-have-you). It's hard for us and our instruments to detect or even be aware of Dark matter at laboratory scales, which operate over the electromagnetic and nuclear forces which Dark Matter feels little or not at all.

At astronomical scales, however, things are different. At those scales, gravity wins out, and is one of the dominant things we observe in astronomy. Dark Matter may not feel the electromagnetic and nuclear forces the way normal matter does, but it feels gravity the same as every other particle does. Nobody gets a pass from gravity, not even Dark Matter.

Consequently, it's relatively easy to observe the gravitational effects of Dark Matter at the astronomical scales of the rotation of galaxies and the dynamics of galaxy clusters, even while it's difficult or impossible to observe the non-gravitational effects of Dark Matter at laboratory scales.