Or maybe, instead of using radio or a golden disc, the aliens decided to synchronize planets of some system (not necessarily theirs), in order to try to communicate intelligent life using the gravitational waves, so the message can reach much further - maybe even to other universes (there are various theories, that we might communicate with other universes in the multiverse using gravity, or maybe with the previous universe before the Big Bang... As well as an idea that the dark matter is another universe that we interact with only gravitationally).