Demons are real, tangible things. Alternate dimensions are real, tangible things, albeit nonphysical (astral) https://en.wikipedia.org/wiki/Astral_plane
Things can happen in these realms, and effect the physical. As well as vice versa. Real tangible spiritual anomaloes have been noted in our existence. One of which is stigmata https://en.wikipedia.org/wiki/Stigmata
Demons exist in an alternate plane, mostly, the astral plane. As long as we deny their existence and deal with them in a logical manner, they will reign. Currently, there are so many demons in our plane with so many hosts, and they seek not to be found. So they will delude their hosts (particularly people of power) into pushing false doctrines and ideologies. This is how demons have gained so , so much power in our world, in a surreal way. That is why jesus himself had to come down and intervene.
Greg von Haesler 