If you encountered a large quantity of antimatter in space, it would annihilate with nearby matter instantly, releasing a huge burst of energy and radiation.

If you encountered a large quantity of antimatter in space, it would instantly annihilate with nearby matter, releasing a massive burst of energy and radiation.

If you encountered a large quantity of antimatter in space, it would annihilate instantly upon contact with matter, releasing a massive burst of energy and radiation.