Espacio Memoria y Derechos Humanos, formerly known as ESMA, was once a notorious detention center used during the military dictatorship in the 1970s and 1980s.
Today it is a museum with a wealth of information detailing the atrocities committed during that dark period in Argentine history. It's a must visit museum, in my opinion, full of information that you likely won't hear elsewhere.
It is heavy and difficult, to be sure. The complex actually houses a number of museums, including a museum for the Malvinas War. But in full honesty, my friend and I couldn't really handle more after our visit to ESMA, it was a lot to process. Go with time.
The plaques are mostly in Spanish but there are audios in English that you can scan with a QR code to listen on your phone via Soundcloud.