At www.dailygalaxy.com/my_weblog/2011/08/could-mirror-matter-be-the-hidden-… (Augusts 4th) … begins with, if dark matter exists it may take the form of mirror planets, mirror stars, and mirror galaxies (a bit of a mirage it would seem). The failure of physicists to actually find tangible evidence of dark matter has led to the idea of 'mirror matter' (basically a spatial reflection of ordinary matter).
Meanwhile, at www.sciencedaily.com/releases/2011/08/110803133519.htm it is suggested, from a paper in Nature August 4th, that a 'model' has shown that a Mars sized object collided with earth and ejected debris to form the moon. However, it may first have formed two moons and the matter of the smaller of the two subsequently went on to collide, very slowly but surely, with the larger moon, in order to create a layer of solid crust that is very thick on one side (its mountainous far side). The simulation showed the smaller moon did not create a crater or caused melt but the material laid down to form a thick layer of solid crust like material. A paper last year claimed the thick crust on the far side of the moon was created by tidal forces rather than impact. In this study the violence of an impact is eliminated by slowing down and widening out the moment of coming together – but is that only possible on a computer screen?
At www.cosmosmagazine.com/news/4581/moons-mountains-made-slo-mo-crash/ which sums up the process. It describes the far side of the moon as having mountains 3000m high with steep craters and gorges.