Of course, it is not density but coulomb potential:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6778505/
"Therefore, whether the experiment is single-particle cryoEM, electron cryo-tomography (cryo-ET), or electron crystallography with 2D or 3D crystals, electron scattering produces maps of the Coulomb (electrostatic or electric) potential in the sample [3]."
Of course, it is not density but coulomb potential:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6778505/ "Therefore, whether the experiment is single-particle cryoEM, electron cryo-tomography (cryo-ET), or electron crystallography with 2D or 3D crystals, electron scattering produces maps of the Coulomb (electrostatic or electric) potential in the sample [3]."
The reference [3] is also useful, it talks about negative density https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5192980/: