Mysteries: Molecular Clouds
A scientist can learn a lot by looking at clouds. Stars like our Sun are born in giant molecular clouds of gas. Some of these clouds, like the Horsehead Nebula, are so thick that they block all light. Their vast stores of mostly hydrogen provide raw materiels for stars and planets. One mystery: Astronomers have no clue how the clouds formed!
The giant molecular clouds are mostly made of hydrogen. One triumph of Big Bang Theory is that it predicts the amount of light elements like hydrogen. Somehow the interstellar hydrogen condensed into protostellar clouds and eventually stars. The clouds are far too diffuse to collapse on their own without dissipating.
The Milky Way is surrounded by a spherical halo of "dark" mass. If this halo is composed of Black Holes, then many times these objects must collide with the galaxy's disk. They would crash into molecular clouds like bullets into cotton candy. Presence of Black Holes would cause portions of the clouds to collapse and form stars.
The Coalsack Nebula in the Southern Cross covers 7 x 5 degrees of sky. In aboriginal astronomy, it forms the head of an Emu in the sky.