Axxman300
Philosopher
I fear that is the nature of ghosts, they don't leave hard evidence.
How would you know? IF they're real we STILL have no idea what we're looking for. We know they're not a gas, we know they're not a mass of some kind of energy (at least not something we can measure or identify), we know their not a liquid, we know they're not a solid. This narrows down the options to a small and crazy list. And frankly the possible list would be made up of particles that only exist for fractions of a second in a super collider, and that is equally silly as the spirits of the dead wandering around.
But...it would be something that could be tested for in the future if someone is rich enough and desperate enough. My money is on the test being negative.
Then again maybe someone will stumble upon something that kicks this phenomenon out of the Halloween store and into a proper science lab My point is that saying that ghosts are beyond science's ability to detect is lazy. Right now science already has explanations which are fantastic in their own right. In the end all things dealing with science comes down to money. You need equipment, involving a major university's physics department along with other science branches is a good idea, you need to commit to a project that might last a decade, and you have to be ready to spend a lot of money only to come up snake-eyes.
Who knows? Maybe someone will point some gadget in an empty room and make a breakthrough. That's the fun thing about science.