Science News notes that:
The heftiest black holes are up to billions of times the mass of the sun and lie at the heart of almost all galaxies. The least massive black holes, which are about 10 times the mass of the sun, form when massive stars blow up in supernova explosions. Mid-sized black holes are thought to form when small black holes sink to the center of a globular cluster and acquire mass.
But, to date, no one has been able to find those mid-scale black holes.
The most recent efforts of astronomers to locate a medium sized one "weighing thousands of times more than the sun" in the places where theory predicts that one might be located, was a bust, revealing only a small black hole. The odd absence of medium sized black holes is not the only interesting size related fact in the science of galaxies and black holes.
Nothing in the theory of general relativity itself compels this result.
As a previous post at this blog noted, the size of the larger black holes is also far from random. The bluges at the core of galaxies are always "500 times as massive as the giant black holes at the hub of their galaxies."
Current theory also provides (in accord with empirical evidence) that "no black hole can become heavier than about 3 billion times the mass of the sun." This also, by implication, places a cap on the maximum size of a galaxy.
Inferred dark matter distributions in galaxies are also exceedingly regular, and can be described with great accuracy empirically by a very simple formula (a modification of the formula for gravity) developed by Milgrom in his MOND theory. No one claims that the simple formula itself is universal or that gravity really works just that way. The simple formula is not relativistic and does not generalize to galactic clusters. But the fact that effect of dark matter distributions in almost every galaxy can be described to fine level of detail with a formula just a couple of lines long is remarkable. There is nothing obvious about the leading cold dark matter theory currently used to explain this phenomena that says that this must be true.
Furthermore "the modern-day universe consists mainly of two galaxy types—young-looking, disk-shaped spirals like the Milky Way, and elderly, football-shaped ellipticals. Ellipticals have a reddish tinge—an indication that they are old and finished forming stars long ago—while spirals have a bluish tinge, a sign of recent star formation. . . . 'red and dead' ellipticals nearly always tip the scales at masses greater than the Milky Way, while the star-forming spirals fall below that weight." This division has existed, at least, for half of the age of the universe.
This suggests, interestingly, that galaxy shape may be a function of central black hole size. While there is nothing terribly astounding about a link between size and shape in physics, there is no obvious reason that there should be two distinct clusters of size and shape in galaxy size either.
Gravity is the dominant one of the four fundamental forces believed to be at play in the phenomena observed by astronomers. The inability of gravity as described in the theory of general relativity to explain the behavior of visible matter is also central to the two great unexplained cosmological phenomena, dark matter, the leading form of which requires a particle not yet disovered by quantum physicists, and dark energy.
Gravity and the related notion of inertia are also associated with the only two particles in the standard model of particle physics, the Higgs boson and the graviton, which are predicted but have not yet been discovered.
Gravity is the only fundamental force for which there is no established quantum description. General relativity and quantum mechanics, while both exceedingly accurate and functional in the domains where they are commonly used, are theoretically inconsistent. The biggest question in quantum mechanics today is how to integrate gravity with the rest of quantum mechanics to create a theory of everything.
One of the biggest "problems" with the standard model of particle physics is that it has large numbers of constants in it that seem to follow some general principles but have no theoretical explanation. Most of those constants are particle masses. Mass, of course, is the property of a particle which describes how it is influenced by gravity and inertia.
Some of science's greatest insights on the nature of time and space themselves are derived from the theory of general relativity, which is also the most accurate known description of the force of gravity.
The gravitational constant is the least accurately known of the fundamental physical constants.
When one starts to look at the major unsolved questions in fundamental physics, all roads lead to gravity. It seems likely to me that all of these questions have a common solution. With the amount of great minds and massive sums of money devoted to solving these questions, we may even find this solution in my lifetime, or at least, in the lifetime of my children. This insight will not be the "end of science," but it may be the end of fundamental physics.