response: There are underground scenes. Thrash metal like Metallica and Slayer became mainstream. Others like death metal or black metal are still niche.
original_response: If you are looking for more agressive, then trash metal bands like Metallica, Megadeth, and Anthrax might be a better choice.
history: I have a new girlfriend who's really into heavy metal music, so I need a quick crash course. Good bands, brief history, that kind of thing. OK, well heavy metal, also known as just metal, began in the 1960s and 70s in the United Kingdom How is that possible? People from the UK just seem so...harmless! Ha.  Looks can be decieving.  In 1968 three major metal bands formed:  Black Sabbath, Deep Purple, and Led Zeppelin. I've definitely heard of the first and third from that list, but I've never really thought of Led Zeppelin as heavy metal. Will she buy it if I mention them? The bands have a number of influences. Music such as those of the blues, folk, and psychedelia. Hm. I never thought blues or folk could really inspire them into heavy metal. Is any other metal types out there for me to talk to her about?
knowledge: Underground scenes produced an array of more aggressive styles: thrash metal broke into the mainstream with bands such as Metallica, Megadeth, Slayer, and Anthrax, while other extreme subgenres of metal such as death metal and black metal remain subcultural phenomena.