These are all questions that display a lack of education in astrophysics. Astrophysicists have put tons of work into investigating and describing star formation and related phenomena. A big enough collection of mass is going to emit some light because the gravitational force will cause nuclear fusion to occur. Note that fusion does not only occur with hydrogen nuclei, and amounts of heavier nuclei are limited by nucleosynthesis measurements from the cosmic microwave background.



Our initial evidence of dark matter were anomalous galactic rotation curves. Some of our best evidence for exotic dark matter is lensing from, e.g., the Bullet Cluster. Based on the former evidence, we know that dark matter is distributed fairly uniformly within galaxies. If dark matter is mostly MACHOs, we should still be able to see lensing from those concentrated bodies within our galaxy, and we don't. Exotic dark matter hypotheses tend to predict a more diffuse collection of particles, so lensing would not be easily visible from nearby sources. The far-away lensing that we observe is not consistent with MACHOs, as Sean Carroll explained in the blog post I linked previously.



Probably half of the physics professors I've known have, at one point or another, made fun of the people in the previous century who said that physics was almost "done." The increasing access to information today means that almost everyone in the field has heard those stories, so that sort of thinking simply doesn't occur as often as it once did in more parochial times.



The latter is, to a large degree, reciprocal with the former. If we know that dark matter acts like matter in general in that it gravitates (which we do) and that evidence has just about ruled out normal matter (which it has), then the obvious conclusion is that dark matter is a new form of matter.