Re: At 90X Jupiter's mass, and fairly pure hydrogen, how did this not ignite into a star?
Brown dwarves are MACHOs, and we have been pretty sure there aren't enough of them to account for the discrepancy for many years.
WIMPs remain plausible, and IIRC the Standard Model even predicts a few possibilities for them. Detection remains a problem as (by definition), they don't interact with the electromagnetic force.
Then there are various possibilities that have yet to make testable hypotheses but might.
And then hundreds of discredited or simply crackpot ones like MOND and others that might fit in some circumstances but do not work for smaller scales - like within a solar system.