This article is from the Red Dwarf FAQ, by Patrick M. Berry pat@interpath.com with numerous contributions by others.
According to the Red Dwarf Quiz Book, it's the remains of Kristine
Kochanski. When Lister comes back to life on the backwards Earth, he
discovers that he's married to an elderly version of Kochanski. Presumably,
treating the canister with gamma and X rays was part of Holly's plan for
restoring her to life along with Lister. (If none of this makes any sense
to you, you haven't read the ending of the novel Better Than Life.)
The original script for "The End" included a scene (cut from the final
version) in which Lister placed the powdered remains of each crewmember in
a separate canister and ejected them all into space (and this is done with
George McIntyre's remains in the aired version). It's reasonable to
conclude that Lister did this, although nothing is said about it in either
the episode or the novel. The number of canisters floating in space
("something in the region of ten thousand") is consistent with the size of
the crew given in Infinity Welcomes Careful Drivers ("eleven thousand, one
hundred and sixty-nine").
 
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