Interstellar Wiki

Cooper notices that Murph's "ghost" is communicating with them.

In Brane cosmology, the central idea in many theories of physics, is that our visible, four-dimensional universe is restricted to just a membrane inside a higher-dimensional space, called the Bulk.

The "bulk beings" referred to in the film are sentient beings able to perceive these higher dimensions, and hence experience time as just another physical dimension of direction. This allows them to create wormholes and higher-dimensional tesseracts (such as the one where Cooper was contained inside the black hole). It's to some extent quite obscure on what it is meant by perceiving FIVE dimensions. But it can be said to be at least 4 spatial dimensions and 1 temporal, allowing for incomprehensible formulations that our infinite 3D + temporal reality cannot fathom.

The tesseract shown in the film was able to manipulate time itself as a physical dimension, thus being capable of showing and interacting with past events. It is alluded to in the film that these "bulk beings" may in fact be humans existing millennia in the future that possess unfathomably advanced technology and have evolved to the point of existing outside of the four (known) dimensions of time and space. However there can also exist another theory that can be equally as fascinating and possible.

Perhaps, before Cooper and TARS had crossed the event horizon there was in fact no hyper advanced civilisation, but crossing the event horizon must also mean that time had STOPPED ENTIRELY. This could mean that from Cooper's and TARS' perspective, they would've travelled an infinite amount of time into the future, possibly allowing for humanity to have advanced within not the span of millennia but perhaps quadrillions of eons of years. After crossing past the event horizon, just within seconds of Cooper's and TARS' perceived amount of time, humanity might have advanced in the blink of an eye to a civilisation beyond space and time, thus allowing them to have constructed a Tesseract in an instant. This could be elaborated further by saying that after Cooper had done his role in the Tesseract, they also could've just sent them both back in time to just the 51 + 23 years elapsed before they had crossed the event horizon of Gargantua.