We do know in fact a lot about the interior of the pyramid. First and foremost, we have detailed maps of the main chambers as they were planned by the Egyptians (a Google search will yield several images of it). But, far more things have been done to the pyramids. In the 1960s, Luis Alvarez carried out a muon tomography of the inside of Chephren 2nd pyramid in order to look for hidden chambers (Alvarez et al. 1970). While the technology of the Alvarez test worked, he concluded that there were not other secret chambers in the pyramid of equal size to those of Cheops and Sneferu. Furthermore, In the 1980s a French team used microgavimetry to look for hidden chambers and passages inside of Cheops’s pyramid. They also concluded that there were not hidden chambers. However, as a fun fact, they left out of their report a curious image that shows less dense areas in a somewhat spiral form (which could in fact be the remains of possible passages). This image could give some support to Jan Pierre Houdin’s theory that the pyramid was constructed using an internal ramp (hence the need for a spiral ramp to move stones up to the top of the pyramid). However, due to the importance of the structure, Egyptians have been reluctant to agree to dismantle some stones on a small room inside a notch on one of the corners of the pyramid to see if in fact there is an interior ramp that was used by Egyptians to built the pyramid.