One of the strangest and most exotic theories to come out of theoretical physics is that the entire universe is a projection of a two-dimensional shell. But the latest evidence suggests the cosmic hologram really is just a crazy theory.
While the notion of a holographic universe opens itself up to all sorts of fanciful interpretations (we had some fun with the idea a while back), the actual theory is fairly...well, "straightforward" isn't really the word. Basically, certain subsets of string theory tell us that it's possible to encode all the information of a 3D volume onto a 2D structure at its boundary.
By extension, the entire universe might just be the 3D projection of information found on a 2D information structure at the edge of the universe. The 3D hologram that we experience is just how the 2D structure is perceived at macroscopic scales or low energy levels. OK, fine...it's still a pretty wild idea, but what it really speaks to is the basic quantum structure of the universe, and certainly not that our existence is somehow less "real" because of it.
Anyway, the idea picked up some support with the findings of GEO 600, a gravitational wave detector located in Germany. The detector found that, at incredibly small scales, there was actually a certain degree of blurriness in the data, as though we were only looking at the "pixels" being projected onto the hologram from the 2D universe.
But now new results have come in from the European Space Agency's Integral gamma-ray observatory, whose instruments are just as precise as those of the GEO 600. The observatory can measure gamma-ray bursts, and depending on their behavior it can determine whether the universe really does become "grainy" at super-small scales. The observatory has measured an extremely bright gamma-ray burst as it traveled 300 million light-years towards Earth, and the results were unmistakable: there were no signs of blurriness.
What's more, we can be sure that there was no blurriness down to 10^-48 meter. That's ten trillion times smaller than the Planck length, which is considered the fundamental unit of length in quantum mechanics. Indeed, it's not at all clear whether it's even possible for there to be distances shorter than the Planck length, at least not in our current understanding of quantum mechanics.
For its part, GEO 600 had detected quantum fuzziness at scales of 10^-16 meter, which is ten quintillion the size of the Planck length. There's no easy way to reconcile these two results without saying that one or the other was in error, and the burden of proof probably has to be on GEO 600, since it's the more dramatic result. It's possible that some exotic factors may affect the gamma-ray photons that were measured by the observatory so that they don't show signs of quantum fuzziness, but that's probably the least plausible conclusion based on the current evidence.
None of this rules out the holographic universe - if nothing else, the holographic principle is still hugely useful to understanding quantum mechanics. But it looks like, if our universe really is a hologram, then the evidence for that exists on scales so impossibly small that they lay beyond our abilities to probe. So then, it's probably best to leave the cosmic hologram as an interesting idea, and - at least for now - not much more.