Ponder this.
Alien travel would have all manner of problems. However, aliens can, and I believe have, travelled here and their presence is detectable in all of us.
DNA. If one can't send oneself, one can send something which will eventually become a clone of oneself. I am a believer in Panspermia, and that this and other solar systems was populated with a primitive DNA. The folks who spread it found a way to send the ultimate message - very rudimentary copies of themselves. This rudimentary DNA was far, far less likely to be degraded by interstellar processes and radiation.
Many plants do the same thing on a local scale. We have met the alien, and he is us.
Arf