How do Densitometer thingies work?

Thank you for all the great ideas for derilect (sp?) ship searching. My
original idea was working around using the passive sensors, especially
the densitometer. I expected that there would be some 'junk' in space, but
(I think) there was generally empty space between systems (can any of you
astronomer types help here?) I figured with a sensor radius of 2 light-years
(for the passive sensors) - something might be worked out.

My biggest question is can anyone tell me how these things 'work'? Do they
sense the 'dimples' in space that masses have? I know they rate them at
High and Low penetration and then xxx kilometers, but beyond that I know
very little. Can they be effectively used in searching for derilicts?

Bart