a legitimate theoretical speculation and possibly valid lines of
actual evidence are given by astrophysicist Robert Foot and a few
collaborators for about a decade for "mirror matter", which interacts
fully with itself, but only about a part in a million with ordinary
matter for electromagnetic forces (chemistry, molecular binding), and
normally via gravity -- so a hunk could be found on the surface of the
Earth, while interacting with the vacuum of the solar system at about
10 deg K -- so to us it would be invisible, heavy, and forever cold --
you could hitch a Sterling engine to it, running from 300 deg K on
Earth to the 10 deg K sink, generating power until (joke) the entire
Earth is cooled to 10 deg K -- perfectly possible physics -- search
Google...
