Personally i choose to believe they exists.. Although there aren't any major scientific evidence that can prove about their existence.. But i heard that some scientist explain its because of frequency of our eye.....

Normally that the "ghost" frequency will be different than out eyes... maybe too low for our eye to detect... When a "ghost" want to appear, they need to absorb the energy in the surrounding to increase their to make their appearance.. ( I donno if this is true or not, just pointing out some of the theory that i think its interesting)

But then, some people say they exists , some don't... But i think human is more scary than ghost =x