Our understanding of galaxies is at a primitive level, and we are still perplexed by basic questions. For instance:
1. Why do things such as galaxies exist at all, with their observed characteristic dimensions—typically 10^11 stars within radii of about 10^4 parsecs?
2. What is the dark matter that constitutes so large a part of galaxies?We know that more than 9O% of their mass is unaccounted for and that the visible content of galaxies is embedded in a diffuse “halo” of dark matter that may be 10 times as massive.
3. Why do the central nuclei of some galaxies flare up (especially at redshifts z=2) outshining the entire host galaxies, and what remnants of this activity are left behind?
Possible answers are now coming into focus as astrophysicists and cosmologists increasingly concentrate on these questions, which are some of the most important ones in physics today.
Galaxies must have evolved ...
