planets (of course) orbit around stars;
stars form spiral arms around the centre of the galaxy;
galaxies form spiral arms around the centre of a metagalaxy;
metagalaxies form spiral arms around the metagalactic centre, from which the monobloc exploded.
Thus, according to this account, there is only one stage of organisation above galaxies: metagalaxies. In Tau Zero, also about intergalactic travel, Poul Anderson has "clans" of "families" of galaxies, although these terms are coined for convenience by the intergalactic travellers who are encountering and traversing these groupings for the first time. Anderson mentions neither spiral arms above the level of galaxy nor any centre from which the universe is expanding.
Although "galactic centre" means the centre of a galaxy, "metagalactic centre" means the centre not of a metagalaxy but of all metagalaxies.
To continue summarising Blish's account:
the system of metagalaxies is a single four dimensional continuum and has an anti-matter counterpart which is a second four dimensional continuum separated from the first by extra dimensions;
in fact, a total of sixteen dimensions is suggested.
Thus, the consistent theme here is "from big to bigger": from planetary systems to galaxies to metagalaxies to one continuum to a multi-dimensional system encompassing at least two comparable continua.
Why do the Hevians and their New Earthmen companions want to move the planet He to the metagalactic centre before the two universes collide? (To be continued.)