The concept of an electron cloud wasn't discovered in a single moment like a new planet or element. It evolved gradually as our understanding of atomic structure changed:

* Early 20th century: Physicists like Rutherford and Bohr proposed models of the atom where electrons orbited the nucleus in well-defined paths. These were planetary models.

* 1920s: The development of quantum mechanics by physicists like Schrodinger and Heisenberg led to a more sophisticated understanding of electron behavior.

* 1926: Erwin Schrödinger's wave equation allowed for the calculation of the probability of finding an electron in a particular region of space. This probability distribution is what we call the electron cloud.

So, you can say the concept of the electron cloud emerged around the mid-1920s with the development of quantum mechanics. It's not a singular discovery but rather a shift in our understanding of the atom's structure.