It is known in the literature that "high-frequency weak limit" of solutions to the Einstein vacuum equations may not necessarily solve the Einstein vacuum equations, but may generate "effective matter fields" in the limit. Burnett conjectured nonetheless that any such limit is isometric to a solution to the Einstein-massless Vlasov system. I will present a recent proof of the conjecture when generalized wave coordinate conditions are imposed. The proof relies on the linear and nonlinear structure of the Einstein equations. This is a joint work with Cécile Huneau.