😬
Seriously, some of you really need to read Crisis on Infinite Earths.
Anti-Monitor absorbed the Anti-Matter Energy so that when he gets back in time, to the beginning, he would still retain the power.
His whole purpose was to travel to the beginning, to change history, so that the Anti-Matter would be triumphant.
And since at the point where he would go back in time the Anti-Matter that provides him with his power didn't exist, he absorbed the Anti-Matter energy into him so that he would still retain his power.