I'd like to think that Lex shared the anti-life equation with Darkseid because he wasn't able to unravel its mysteries by himself. Darkseid was actually a composite of 2 beings (he also had Brainiac technology incorporated into his resurrected form). And we know that Brainiac can lie dormant in a host body. Thus, at the end, we have all 3 major villains - Luthor, Darkseid, and Brainiac, - all getting their hands on the ultimate power (the anti-life equation).
One theory is that since they are all 12th level intellects they were able to ascend to a high level of power. However, the fact that Metron allowed this to happen makes me think that he is the true hero of this series. Metron found a way to rid the universe of its 3 most dangerous villains by allowing them to simultaneously obtain the anti-life equation while at the same time knowing that none of them could handle it. Otherwise, why share this information with Luthor? He could have shared it with Mr Terrific, the Atom, Brainiac 5 or some other more benevolent genius intellect.
I interpret the ending to be similar to that of Raiders of the Lost Ark. The bad guys gazed upon ultimate power and were promptly absorbed into the Source Wall.

Even Lex Luthor couldn't get one over on Darkseid though. He's in a much higher class to Luthor.

Let's see Lex Luthor offers Darksied something, "the only thing", he really wants. Lex Luthor hmmmm.
All I can say is whatever happened to them what Lex had to offer had a catch. In a situation like that Lex ALWAYS wants the upper hand. And he never give anything without SOMETHING in return.

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It feels like it was left open, they could always come back in the future.
QUOTE
:
Time can be generous but ultimately time is indifferent.

Maybe it made it so that both Luthor and Darkseid didn't exist any more.
Yippee: "For king!"
Yappee: "For country!"
Yahooie: "And, most of all, for 10 an hour!"