Well, they are purely gravitational.
I should have said matter instead of mass, actually. 
There is a subtle difference...
 
Actually, 
with Kaluza-Klein theory you can completely describe a universe with gravity
and electricity and magnetism (light) using 
only the theory of general relativity (gravity).
		
		
	 
 
The question was:
 
What are phisical parameters of ' non-trivial universes without mass ' ?
 
The answer was:
 
Well, they ( parameters ) are purely gravitational. 
I should have said matter instead of mass, actually. 
There is a subtle difference...
Actually, 
with Kaluza-Klein theory you can completely describe a universe
with gravity and electricity and magnetism (light) using only
the theory of general relativity (gravity). 
===..
 
My conclusion from Niflmir answer:
 
Hmm.
I asked the parameters of ' non-trivial universes without mass ' 
and Niflmir answered :
. . they ( parameters ) are purely gravitational ( 
Matter / Mass ) 
. . . 
with Kaluza-Klein theory you can completely describe a universe
with gravity ( 
Matter / Mass ) and electricity and magnetism (light)
using only the theory of general relativity (gravity). 
=.
 
But which theory I need to know in order to understand 
the parameters of ' non-trivial universes 
without mass ' ?
 
!!!
===...