Dark matter is made of tiny black holes

The argument

Scientists postulated that dark matter can be made of big or tiny black holes. Research has discovered massive black holes millions to billions times the size of the sun. These black holes are the easiest to detect because of their large size. Detection depends on the effects an object has on what surrounds it, and a greater mass produces larger and more readily observed effects, so the population easiest to find is also the one about which the most can be established. Theoretically, there are many smaller black holes that formed after the big bang but are undetectable due to their small size. The same reasoning runs in reverse here: an object whose influence on its surroundings is slight will not announce itself, so an absence of observations is not evidence that such objects are absent. Scientists predicted that dark matter can be made of the massive black holes, the tiny black holes, or a combination of the two. Those three possibilities exhaust the options, which means evidence against one of them strengthens the remaining alternatives rather than undermining the family of hypotheses as a whole. The data on massive black holes did not support the hypothesis that dark matter can be made of big black holes because of their scarcity. This is the decisive step in the argument. Dark matter is required to account for a very large quantity of mass, so any candidate must be abundant enough to supply it, and a population that is observable and turns out to be rare is ruled out by the very observations that established its existence. Tiny black holes are still candidates of what constitute dark matter. They survive the test that eliminated the alternative, because nothing about their abundance has been excluded by observation. Dark matter is made of tiny black holes and research on dark matter should focus on discovering them.

Premises

Counter-arguments

Most modern scientists disprove the theory of black holes as a component of dark matter. Scientists conducted experiments involving the detection of gravitational lensing: the phenomenon of light bending as an object like a black hole moves past another due to gravity. Gravitational lensing results in an increase in brightness of nearby stars which researchers can detect. If an increase in brightness of nearby stars is detected, then a black hole has passed. Scientists predicted to see 1000 brightening events if dark matter is made of black holes. Scientists detected only one event. The results showed that black holes can contribute no more than 0.1% of dark matter. It is highly unlikely that black holes make up dark matter.

Rejecting the premises

[Rejecting P1] Accurate as a description of what has been proposed, but the premise sets up a false choice. Primordial black holes are one candidate among several — weakly interacting massive particles, axions and sterile neutrinos among them — so ruling out one option does not leave the other standing as the answer. [Rejecting P2] Correct, and the same class of observation constrains the small end too. Microlensing surveys look for the brightening of a background star as a compact object passes in front of it; a population of primordial black holes sufficient to account for dark matter predicted on the order of a thousand such events, and one was detected, bounding their contribution to roughly 0.1 per cent. [Rejecting P3] Undetectability is not a point in the hypothesis's favour. A candidate is supported by evidence for it, not by the absence of evidence against it — and small black holes are not in fact unobservable, being constrained by lensing surveys, by the cosmic microwave background and, at the smallest masses, by the evaporation the theory itself predicts. The premise treats difficulty of refutation as a reason for acceptance.