4th Dimension?

Somewhere, somehow, all astronauts, astrophysicists and all other 'astro' people seem to somehow come to the same confusion... They know the 1st Dimension. They know the 2nd Dimension. They know the 3rd dimension but... 4th Dimension? There's got to be one, but what is it?
Here's what I cooked up...
What if the 4th Dimension was just nothing but nothing? Funny isn't it? We've never, even scientists themselves, have never, never seen, felt or studied nothingness. My theory could be wrong, but no one, living  right now can prove anything related to this because we need a solid theory with proof to explain this unnatural dimension.
Here, in the Earth, we're always surrounded by air, dust, dead skin particles, smoke, water vapor and all floating substances which makes us completely hidden from this dimension, but yet we know there is something, something which humans are not meant for... for now.

It is vague, unexplained and completely unknown, so, I want YOU to help me, and whoever reads this, by giving suggestions, corrections or even oppositions!!! Make this site the discussion forum. Tell your friends, relatives and everyone because even one of a million could have a true million dollar idea...