Quite correct, a solar flare from Betelgeuse will (probably) eventually reach Earth.
Photons of light from Betelgeuse obviously reach Earth otherwise we wouldn't be able to see it. Solar flares contain charged particles, and there is no reason why these won't eventually reach Earth as well. Interstellar space isn't empty, it does contain a very low density of matter, and this can interact with charged particles. See http://en.wikipedia.org/wiki/Interstellar_medium for details. In principle dark matter would interact with the particles generated by solar flares, but in practice the interaction is so weak as to be negligable.
Note that in some places the interstellar medium can get quite concentrated and in fact can be thick enough to block light. These are generally called nebulae, and these will absorb particles from solar flares. However in general a charged particle from a solar flare would travel a very very long way before it hit anything.
Although they probably don't come from solar flares, the Earth is hit by cosmic rays, and some of these are thought to come from distant galaxies. This shows that charged particles can travel for very great distances.