Actually, the word for perception, kathoratai, functions as the main verb of Romans 1:20, and it is in the present tense. Literally, it is "are being perceived," or "are discerned clearly." The english perfect tense rendering of the ESV is still consistent with this, and you cannot get "did" out of it (also see the AV). In the translation below, "being understood" is also in the present tense. "Because that which is known of God is plainly evident in them, for God made it clear to them; for the things which are invisible of Him since the creation of the universe are clearly seen [perceived], being understood by means of the things that are made, namely, His eternal power and divinity, resulting in their being without a defense."