Einstein must have known by 1920 that the aether, and the aether drift, had been proven by the Americans Michelson, Morley in 1887; Morley and Miller from 1900-1905; and Miller from 1905-1913 with hundreds of thousands of light interference readings. Einstein took the aether out of science in 1905 with his ‘Special Theory’ of Relativity but as any good magician is wont to do, the great philosopher reinserted the aether into his 1915 ‘General Theory’. Given the aforementioned light experiments and mechancial proofs, he had no choice.

This aether ‘resurrection’ (Einstein’s word) was an ‘imponderable’, largely mass-less, pseudo-gaseous entity, not of any great materiality or mobility (see quote above). He used it to explain ‘gravity’ within his 4th dimension of the unproven spacetime merger. This conjuring was of course a philosophical trick, it was not based on experimental evidence.  More here