http://arxiv.org/abs/0909.3456 Antigravitation Sabine Hossenfelder (Submitted on 18 Sep 2009) We discuss why there are no negative gravitational sources in General Relativity and show that it is possible to extend the classical theory such that repulsive gravitational interaction occurs. ***** This anti-gravitation is not due negative mass, but negatively charged gravity. So, this is a kind of mirror matter with a repel in relation to our normal matter. They'd cluster in big voids and would interact just gravitationally with our normal matter. IMHO, that could be related to cosmological constant, given the repel of clusters of matter and dark matter and mirror and dark mirror matter. So, I was thinking of what could be dark matter. Perhaps matter with euclidean signature, in which they'd just communicate geometrically with normal matter, not even gravitationally, given that none of the known forces can yield propagating fields (waves) in a euclidean signature, as far as I know. What do you people think? Has anyone seen something like this? I have seen a few times, that this is the nature of the annoying scalar field in Horava gravity.