we would not be unique in terms of sin and death entering into creation. There would have to be other Gardens of Eden
The question of aliens is uncertain. The Bible is all about THIS creation - our universe and its time. And the Bible doesn't tell us everything, likely because either God doesn't yet want us to know about them or He considers us unready for such knowledge or, at least for us, such knowledge is irrelevant. He gave us what He believes we need to know, for NOW. IF He has created other beings in our universe (and I don't have any reason to believe so), He may have placed them so far away as that we will NEVER encounter them - meaning present knowledge of them is unnecessary and unneeded. IF He created other beings, He may deal or have dealt with them differently than us. Does God deal with the angels the same way as He does us? Are we to believe that the angels not part of the rebellion have never, EVER sinned, not even once - as only GOD is Holy and perfect? God's other created beings may be like angels or they may be as different from angels as we are from angels. Who knows? There may be another special place of punishment for aliens - or a specific segment of hell designed for them. Again, who knows? We can only go on what He has told us.
As for "other Gardens of Eden," God could have created other realities and universes differently. They MIGHT exist or might not, separate and undectable by us. God MAY have created other universes and beings that no longer exist. Other times may have come and gone. God may have created in other spiritual planes or dimensions, still existing or no longer. We have to realize that God is capable of anything that is not outside of His Holy Character. He can create anything and in any way He so desires. He is also omnipresent - can be in many places all at once. No space or reality can contain Him, as He transcends all He creates.
We also have to remember that all God has created is NOT all about US and OUR time. One of God's major attributes is that He is constantly active, creative and at work. He is an ETERNAL being without any beginning or end. Time is a mere tool and creation of His. And so even if our universe is nearly 14 billion years old (or whatever it truly is), there has been an enternity of God's activities BEFORE our time and the creation of OUR universe. And so the question is, what might that eternity have included? What activities, creations and beings did God
possibly create? Were there PREVIOUS heavens (principle spiritual abodes of God)? Out of a endless, previous eternity, why would or could we be certain that God has only created one universe, EVER? Only one planet of beings, EVER? We know that God was active in eternity past - which means the possibilities are uncountable, and we shouldn't be surprised at what God might have done. It's pure human narcissim to exclaim that our reality is the only one (besides angels, Heaven, hell, etc) that God has ever created. We can only speculate and know what God HAS told us. And Scripture tells us that God DOES have secret things that we do not yet know.
“The secret things belong to the Lord our God, but the things that are revealed belong to us and to our children forever ..." (Deuteronomy 29:29 ESV)
"It is the glory of God to conceal things, but the glory of kings is to search things out." (Proverbs 25:2)
“Can you find out the deep things of God? Can you find out the limit of the Almighty? It is higher than heaven—what can you do? Deeper than Sheol—what can you know? Its measure is longer than the earth and broader than the sea." (Job 11:7-9)
