How can I justify plain text in Linux?

you need par (official website, source code, package status on Repology)

e.g. to get it on ubuntu, do:

sudo aptitude install par

to justify text.txt (at the width of 80 chars), saving in newtext.txt:

par j1w80 < text.txt > newtext.txt

to use par in vim or gvim:

:set formatprg=par\ j1w80

then highlight the text you want to format and use the gq command.

for more information, see man par

fmt is the old-school solution

From the man page:

fmt [-cmnps] [-d chars] [-l num] [-t num] [goal [maximum] | -width | -w width] [file ...]

and

The goal length defaults to 65 and the maximum to 10 more than the goal length. Alterna- tively, a single width parameter can be specified either by prepending a hyphen to it or by using -w. For example, fmt -w 72'',fmt -72'', and ``fmt 72 72'' all produce identical output.

You can do this in emacs using fill-paragraph or fill-region (fill docs).

You need to pass a numeric prefix argument. The default fill column is 70.

Select the text to fill and do M-3 M-x fill-region.

I set the fill column to 53 for your sample text and got:

These phantasmic balls  have some strange properties,
some   neat,  some   interesting,  and   others  just
indicative of lazy  design. They're almost all caused
by  the same design  flaw: The  game treats  them too
much like regular balls.