My hair colour is a very bright red, and I want it to go much darker without dying it... so please help?

If you wash your hair with dishwashing liquid, it will SLOWLY fade the color. It will make the hair go lighter and yellower (oranger).

If you want to go much darker, I don't think you have any choice but to dye it. There are lots of dyes -- permanent ones, semi and demi-permanent ones and temporary dyes. One of those may suit your bill.