One theory of dark matter is that it's strange quark antimatter. An asymmetry in the behavior of quarks and antiquarks in the very early universe would have led to antiquarks being preferentially squirreled away in tiny ultradense nuggets of quark matter. While explaining dark matter, this would also explain the observed matter-antimatter asymmetry. Or, rather, it would explain it by saying matter and antimatter are present in equal amounts, but the antimatter is in a different form.

If this theory were true, tiny nuggets of this antimatter would be passing through the solar system all the time. Perhaps a future society could detect them and somehow trap some for use as an energy source.