According to theoretical physicist Juan Maldacena, the universe is a hologram and everything you can see - including this article and the device you are reading it on - is a mere projection. In other words, there is a two-dimensional surface that contains all the information needed to describe a three-dimensional object - which in this case is our universe. That data containing a description of a volume of space - such as a human or a comet - could be hidden in a region of this flattened, 'real' version of the universe. Continue reading for a video and more information.
Here is a bit more: "In a larger sense, the theory suggests that the entire universe can be seen as a 'two-dimensional structure projected onto a cosmological horizon' - or in simpler terms, the universe we believe we inhabit is a 3D projection of a 2D alternate universe."