[bojan]

Well, if it is not EM, it could be sound....
http://hetdex.org/dark_energy/how_fi...und_waves.html
http://www.newscientist.com/article/...lustering.html
http://www.newuniverse.co.uk/archive_1766_print.html

http://arxiv.org/ftp/physics/papers/0512/0512047.pdf