Using then instead of at that time

[Sundar Naz 4]

The scientist reached the conclusion in 1990, but it was ignored at that time.

Can I use then instead of at that time in the above sentence? 

[PHIL73 89]

Hello Sundar,

I think both are correct. 

You could also use "In those days" or "In those times" as it is an event which took place a long time ago.

I hope it helps (and I hope I'm not wrong )