If they have minor, persistent effects. So technically within our perception, but not really noticeable. If some force of nature made your shadow .1% shorter on

If they have minor, persistent effects. So technically within our perception, but not really noticeable. If some force of nature made your shadow .1% shorter on even-numbered Thursdays, it would exist but be outside our perception for some value of perception. Something whose existence cannot be observed or falsified doesn't 'exist' in any meaningful sense, but I suspect that there are things that exist, in a meaningful sense, even though they won't be noticed.