We may live on a three-dimensional braneworld, floating in a higher dimensional space. String theory requires there are 9 + 1 dimensions of spacetime, but our world seems to only be 3+1 dimensional. So where do the 6 extra space dimensions go?
Originally it was thought that these extra dimensions would 'curl up' so that they were of very small size (Planck scale) and thus would not be visible. This idea is known as compactification. There was a great deal of excitement created in the 80's when it was discovered that these extra dimensions should be compactified on a compact manifold known as a Calabi-Yau manifold, and that the number of generations of quarks and leptons could be correlllated with the number of holes in the Calabi-Yau.
However, with the second string revolution circa 1995, objects called D-branes were discovered that can provide an alternative to compactification. The essence of D-branes is that open strings are attached to them. That is to say that the ends of the strings satify Dirichlet boundary conditions, from which comes the D in D-brane. Since the strings represent matter and they are fixed to the D-brane, this means that this matter would not be able to experience the extra dimensions. For example, the photon is one type of string excitation. So photon on the D-brane would not be able to leave the brane, and thus would not be able to illuminate the extra dimensions. Our world may be such a 'brane world', and three-dimensional island floating in a higher dimensional space.
