This article is a spectacular read, or at least I thought it was.
http://scitation.aip.org/content/aip/journal/adva/6/6/10.1063/1.4953807
in short:
The Hypotheses: When two oppositely polarised photons line up their spectral emissions and location, and hang-out together, the bits of them that are measurable by normal methods essentially get cancelled out, making them appear not to be there. What we normally consider to be a perfect “vacuum” is however full of them. 
Together this “photon pair” becomes utterly invisible to just about everything, except a teensy-tiny bit of mass so small its only measurable at gravitational scales.
Result of this Hypothesis is that:
1 - “dark matter” can be explained as a bunch of photon-pairs chilling out in space.
2 - the proven fact that the sum of the gravitational energy of two large objects is different once they are combined to a single object is explainable by the fact that they had to dissipate photon-pairs while doing it
3 - “inertia” can be explained as “photon-pairs being in-the-way”, and having to move them out of the way takes energy.
4 - the “Electromagnetic drive” that baffles scientists by emitting no “exhaust”, but actually works, can be explained as a device that injects random single-photons ( microwave) that are reflected, but due to its design actually brings them into alignment, and pairs-them-up into photon-pairs before they are emitted , resulting in them being un-measurable using normal methods.
6 - a bunch of other really cool things are explained, and no existing fundamental laws are broken.
http://scitation.aip.org/content/aip/journal/adva/6/6/10.1063/1.4953807