So where are all these extra dimensions?
A new report out of Stanford has now made an even more stringent test for Newtonian gravity all the way down to 10 microns!
It has been proposed that if there are extra dimensions for gravity to propagate, then at some very small distances, gravitational strength will start to deviate from the known Newtonian description. Originally, there were proposal that this deviation might be at the millimeter scale. That has been blown out of the water. There has been progressively smaller and smaller length scales over which gravity has been checked, and this latest report is the smallest as of now.
So really, where are these extra dimensions already!
 A.A. Geraci et al. Phys. Rev. D v.78, p.022002 (2008).