Subject: does time dilation add in series?

I read that time dilates and light appears to travel slower when it passed 
through a strong gravity field.  But does that affect accumulate if a light 
particle has to pass into and out of numerous gravity fields. Like when light 
leaves a quasar it climbs out of the high gravity and redshifts, and then gets 
lensed around a black hole, would it redshift again?  Would the presence of 
dark matter be able to explain away the apparent expansion of the universe if 
the universe were uniformly occupied by dark matter, thus making light from 
large distances dilate proportional to distance due to gravity, rather than 
redshifting due to expansion.

Re: does time dilation add in series?