Idealism is the taking the step of letting go of the mystery ballast that is deemed to be outside of mind in the large sense. It seems like a step that you take but it is no such thing since you are already floating freely in mind whether you know it or not, and are no more than the element in which you find yourself as the element is no more or less than you. Perhaps then it is just the realisation that mind, again in the larger sense, is dense enough, necessarily dense enough, to maintain buoyancy whatever happens, not having an outside. The so-called ballast then was just an assumption needed to make the old dualistic scheme seem plausible, in other words a certain exercise in meaning and no more. There could never be evidence for such dark matter, as evidence (there to convince who?) it would have already contradicted itself, being only a prejudice in favour of an arbitrary limitation of mind's scope, of meaning's scope, nothing outside of what it means in each and every relation and nothing inside as well.