I have seen some people arguing if DU is really infinite, and i honestly don't know, but in theory true infinity could be achieved like this:
minecraft isn't infinite because the world center is the center of the map, and thus due to floating point exceptions, the extremes become jumpy. additionally, the terrain generation has been fixed, but before that it was wired because of floating point exceptions too (and an error in the algorithm).
recipe to make a truely infinite universe:
1: the center of the universe is literally the player, he will thus always experience the optimal floating point precision.
2: each planet/chunk must have its own plane of reference. no float as position, no double, Big Integers. they will only become expensive once in the 128 bit range: 1.7014118e+38 units, which for 1mm precision, you would have 1.7014118e+35 meters with the galaxy being 9.4607e+17 meters which is 1.7983995e+17 galaxies, so yeah.... try travelling that in a lifetime.
3: have planets in their own frame of reference and organized in an octree lookup table so you can easily get the reference described of all the planets around you so you can query them individually for different precision. lower precision are not generated until needed (the metadata is kept with the planet to resume generation of more detailed features.
4: every enclosing type has an octree of the types it encloses to promote efficient movement (galaxies could rotate efficiently with its people and planets inside)
the Intel Xeon Phi already even supports 512 bit operations... so yeah... performance is not an issue...
this system allows for a linear performance scaling in performance and memory needs of the world.
I don't think NQ has implemented that, since 64 bit sizes already allow for a galaxy sized playing fields, but if they are gonna be around a long time (10 years) they better plan for a system, to use the available performance for optimum fun