Let's just say that some people are more supseptive to spirits, and can therefore see them....
I know for a fact, that they exist....My sister has seen many, she is very "spirituallly" gifted that way...Others I know, also have seen ghosts....I do not doubt these people.
Gas, like propane, or natural gas is a substance, but it cannot be seen. does that then mean that gas does not exist? We know little about the spiritual world, only what we can see with our eyes.
Did you know that Ultra violet light exists, light that our eyes cannot see.? But it is there.
Gravity is also here, but I don't know for sure, if it could be said to have a substance.....does it?
Radio waves and television waves are in the air all around us...can we see them, do they have substance..? No.....No substance.... We know they are there, though. If someone told us, and we didn't know, we would doubt the person telling us...we would think he was crazy.
Believe in them, for they do exist.