Dark Matter and the spiritual realm

[Welsh cake]

I was watching a program about cosmology with my spiritualist wife when the program mentioned that there was matter that we cant see or interact with that accounted for 90% (I think it said) of the mass of the universe.
My wife suddenly became quite excited, and said 'couldnt that matter be the spirit realm' and more or less dared me to prove somehow that it wasn't.
A heated discussion ensued that ended with her being less than convinced that I was right.

Help.

What is spirit supposed to be anyway? As a former Christian I always thought it was energy, or something physical that science simply can't detect (please don't laugh) but there doesn't seem to be any reliable definition among religious groups over what the hell it, and/or the realm made up of it, are essentially.