it's almost certainly a garden escapee - either cross between a regular elder and a purple elder (Sambucus nigra f. porphyrophylla to give it its posh name), or a not-particularly-special purple elder that's seeded itself or suckered or just been dumped by someone disposing of garden waste.
Proper purple elder has very pink flowers and deep purple leaves as Rosemary said and is a really ornamental plant. It makes great pink champagne but is otherwise exactly the same as regular elder. Both have black berries.
I'd imagine your serrated-leaf elder is another sport of a regular elder - there are lots of different species, hybrids, forms & variations. You could try patenting it and selling it for millions as the Next Big Thing in horticulture but the lack of flowers mightn't appeal to gardeners!