HELP - Electrical / Wiring with new build of home

Hello and thank you for reading this.

As some of you know, I am in the process of building a new home, with a dedicated audio room, 5 dedicated 20A circuits, etc., and other features to hopefully help the system sound its best for many years to come.

While walking the house this evening, I noticed the large "umbilical" coming from the breaker box, running through the basement rafters out to the garage (I assume to be connected to the meter outside) appeared to be aluminum. The electrical drop must have been a good 2" or perhaps even 3" in diameter, with 4 distinct stranded wires inside. Again- all appeared to be aluminum.

I have done some forum searches, and didn't find much, but am quite sure I have been told to make sure it is all copper and NO aluminum. I have already shot off an email to the builder, but I will need help and/or "ammunition" as to why the aluminum is "unacceptable" and why it needs to be copper.

Please correct me if I am incorrect and the aluminum is fine. Please also give me all the info I need to get the copper if the aluminum is not good.

Thank you for all the help in advance!