Did mass, that’s also understood (8)
I believe the answer is:
gathered
I believe this is a double definition.
'did mass' is the first definition.
(to gather is to assemble or mass)
'understood' is the second definition.
('gather' can be a synonym of 'understand')
'that's also' is the link.
(Other definitions for gathered that I've seen before include "assumed" , "Surmised" , "Collected - inferred" , "Picked up" , "Concluded from evidence" .)