[brabel]

Not to mention that when we talk about other galaxies, then distances are on a whole different scale again. While travelling to other stars takes "only" a few thousand years, to go to another galaxy you might need several hundred million years with any technology we can currently even imagine.

Even if intelligent life is rare enough that it only appears once on a single galaxy every several billion years, we're still talking probably millions of possible civilizations, though each almost completely out-of-reach of each other.

[pfdietz]

We can achieve relativistic speeds with technologies we can imagine (specifically, laser light sails). This would require just millions of years to get to M-31, for example (which is 2 million light years away).

[ncmncm]

Arriving not in atomically finely tattered shreds is left as an exercise for the descendants. And, stopping.

[pfdietz]

They'd presumably do this starting from the fringes of the galaxy where the gas is thinner, and with a sufficiently big vehicle (perhaps assembled out of parts that are accelerated separately) that it could withstand some erosion. The sail itself would be discarded after acceleration.

Drag on the interstellar medium using magnetic fields could be used for deceleration.

[ncmncm]

Standing up to millions of years of relativistic erosion seems difficult.

Accelerating ISM ions near your destination to slow down would necessarily consume as much energy as it took to get up to speed, and without assist from folks at home (who are anyway a different species by now). You will have plowed through untold tons of relativistic IGM on the way that I suppose you could collect on the way and then fuse to power your EM brakes.

Retaining all that IGM seems mainly a matter of dissipating heat, provided you have discovered how to keep it from instead blasting bits off of your bumper. It would, also, all need to be accelerated to match your speed. A substantial fraction of it would be neutral, so purely ballistic.

Maybe you have a million (or ten; hundred?) km of very diffuse spray foam for your front bumper, that you will wring out for protons when time comes to stop?

Presumably you want to stop where whatever you made the trip for is, assuming it is still there ("lo!") those millions of years later. It must be something you couldn't find anywhere among the half-trillion stars back home. (You did look, right?) Or maybe you just wanted not to be there -- neighborhood gone downhill, galaxy too small for both you and the ex?

Given that you can weather a million years of relativistic battering, and actually collect industrial feedstocks from it, maybe you don't really need a galaxy or anything in one anymore, and can just keep going. It ought to be safer out there, without an ever-present threat of magnetars around always threatening to belch slightly and sterilize everything for much farther than can be seen without a telescope.

[pfdietz]

Accelerating ISM ions near your destination requires no additional energy at all -- it's just drag. The kinetic energy of the vehicle suffices.

The Local Group is embedded within the WHIM -- Warm/Hot Intergalactic Medium. This is a hot, ionized gas with density maybe a few ions per cubic meter, maybe 100,000x less dense than the ISM in our galaxy. Even better, it's ionized: a magnetic field will enable the ions to be guided around the vehicle without any erosion whatsoever.