As best we can tell, ghosts do not exist. They are likely the result of overactive imaginations and myths that people spread which only encourages the belief. No study into the phenomenon has ever resulted in actual proof of their existence that can be measured.
The ash ghosts of Valencheirn seem to be a different story, but no one as of yet has an explanation for the cause. Some theorize it has to do with them dying without realizing that they were killed. Perhaps if they had been aware of their deaths they would have survived.
wackymon said:Near Ness, they could hear a mild "Phshooooooooooooooooo" sound... As two peculiar figures appeared.
"Well... It appears our transportation from your world to the enterprise met with a mild Difficulty..." he said, "I'll run a system diagnostic quickly, so as to determine whether we should be worried."
"SSssSS!" the creeper said from the ally nodding it's head... Then slowly turning its head to Ness...
"Ssss. SSSS! It said, tipping it's head to Data, like it was asking him something.