Everything visible reflects light, so if they are not reflecting light then maybe they are light.
I have never seen ghosts nor subscribe to belief in them, however I do subscribe to various PSI events such as clairvoyance, and telepathy.
I do not rule out their possibility.
If Clairvoyance is real as I have witnessed/experienced firsthand (in my opinion) then time both future and past may be an illusion. Current science does not offer PSI explanations to which I feel must exist so I have been forced to look at other ideas and theories in this quest.
I think our actions today can alter our past/history in the same way a real cat in Schrodingers box would affect the past (moment of life/death) when the box is opened. I think the outcome would be based upon expectation/faith/belief of those observing and involved including the expectations of wigners friend, and wigners' friends friends until you introduce people who are unaware of the mechanism inside schrodingers box.
So.. Ghosts could be.
A) ourselves or people we might see during a different possible history/timeline.
another possibility is they imprint emotions upon walls or places and the walls/places repeat the scenario. Maybe if a person was being chased with a gun after seeing her family murdered the fear might implant itself in surrounding matter. so..
B) Surviving strong emotions captured by walls/place being re-lived to any in tune.
or simply they are spirits not wishing to move on as is the popular belief.
C) disembodied spirits either unaware of death, or too happy to move on, or wanting to fix a wrong.
There is high amounts of energy in the smallest particles of dust. Do not forget that dust is moving at great speeds as the earth turns and travels through space it is moving at over 1000 miles per hour. Can they use this energy?
What about perception? Can everyone see the same ghost or does someone need to be "sensitive"
Too many questions, too many variables, too many possibilities, and no clear answer.