Incurable optimism makes them possible. They are desirable in order to avoid extreme vacuums or extreme densities less we (or they) intervene. We are becoming an "inconceivably advanced civilization" and unless the universe acts soon it won't be able to terminate us.

We can move stars by using robots capable of emplacing reflectors for each sufficiently near the hemisphere in the direction we wish to move it (converting the star into an ion rocket engine). Gravitational coupling will bring along its gas giants and other planets. If we can move star systems, by synchronizing the movement of stars, we can move a galaxy (I hope the black hole of each can be brought along via gravitational coupling). By synchronously moving galaxies we can change the configuration of clusters as well as move them. Thus we can proceed up the scale as far as we wish as time permits. Our current level of technology is capable of doing this today, except for black hole unzipping; however, I hope we can improve it by several orders of magnitude.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?