There aren't Ghosts, but there are Negative Energies
So here comes the concept of Negative Energies. Negative Energy is not something such as smoke as you can see above. It is something that is invisible. It not only makes the environment negative, but it also affects the people in that particular environment. Like, for example. If there is a source of this Negative Energy, then it will spoil the mood of a person, making her or him much tensed, and sad, vexed and irritated. And the result is that everyone is vexed and tensed and annoyed, and no one will talk to each other. And no one will interact. And this makes the environment much more worse, and making the Negative Energies Stronger. But now, you would say that from where does these negative energies come from.
Such energies come from angry or negative people, things and surroundings. For example in a house, there was a fight between two brothers. So that particular environment would become negative. Its a saying, that if you don't make your bed and clean your room properly, then Ghosts dance on your bed. Obviously they don't exactly dance on your bed, but Negative Energies do come in the particular place.
We can even create Positive energies by Bhajans, Peaceful Music, Happy and Clean Environment, and such things. Hence you got one more moral. Never leave your bed and room not cleaned otherwise Ghosts will dance on them! Just kidding! See you soon!