Here's my teaser.

The Michelson–Morley experiment found no evidence of a luminiferous aether and Einstein's Special Relativity explained the mechanics of light, obviating the need for a medium (luminiferous aether) for light waves to travel through.

Since then other hypothesis have been posited for a luminiferous aether but none have been elevated to the level of theory. There is no independent and repeatable experiment/observation to support these hypothesis.

This is not to say it doesn't exist but the same can be said for any speculation lacking concrete observations supporting its premise and predictions.