In my garden a plant just popped up randomly and I don't know what it is. It came last year in my garden, and I was wondering till then. The leaves are dark green and the underside is a but purple as in the picture:
This looks very much like an avocado seedling. It happend to me last year that I had some avocado pits in my compost bin, and when I used the compost to mulch my garden the pits were still viable. From your comments I think it is likely also the case here. If you live in a temperate climate the plants will probably die somewhere in winter. However, I live in North-western Europe and my seedlings actually survived a pretty cold winter, so they are still in my garden now. They died back to the ground, but new shoots emerged late spring.