In the early 1900s, many physicists were skeptical about the existence of atoms, a belief that persisted despite earlier contributions like Mendeleev's periodic table and Avogadro's number. Einstein's work on Brownian motion in 1905 provided significant evidence for atomic theory by demonstrating how fluctuations in atom fluxes could explain the movement of smoke particles. This marked a turning point, as it shifted the debate towards accepting atomic theory, although it was not an outright proof. The discussion also highlighted the historical skepticism from figures like Max Planck, who initially doubted atomic theory due to its implications for thermodynamics. Ultimately, by the 1900s, the evidence for atomic existence became widely accepted among physicists, despite some lingering doubts.