When you ask "Atheists, God is presumed by many to have created this universe. Explain, in detail, your alternative hypothesis on how this universe exists?," I presume you are referring to the god of the Bible.
Well, the Bible makes it clear--from the descriptions in numerous passages--the nature of the cosmos that the biblical god created.
That cosmos is NOT the universe that we know of, and, in fact, that cosmos does not exist because that cosmos is based on ignorance and primitive myth. Since that cosmos is an attribute and manifestation of that god, then that god does not exist either.
Note: Because only a few verses would not make the case, the material below is necessarily quite extensive and shows that the Bible has a consistent view of the structure of the cosmos throughout, with parts that are fully consistent with each other. That structural consistency indicates that it accurately represents the cosmos as conceived by the writers of the Bible.
According to Ecclesiastes 1:5 the sun goes (hasteth) around the earth
"The sun also ariseth, and the sun goeth down, and hasteth to his place where he arose"
--as, of course, it must, since, according to Ps 93:1, Ps 96:10, and 1 Chr 16:30, the earth does not move. And the earth cannot move because, according to 1 Samuel 2:8 and Ps 75:3, it is placed on pillars. And because it is placed on pillars, it has an underside and an upper side, as confirmed by Isaiah 40:22 which indicates that the earth is a circle--i.e., a flat disk.
That is also confirmed by Proverbs 8:27, which describes god as beginning the creation of the world when he "drew a circle on the face of the deep" (ESV). The Hebrew word translated as “circle,” "compass, “ and “horizon” in the different Bibles is the same word used for circle in Isaiah 40:22. The ancient Hebrews would have gotten the idea of a circular earth by viewing the horizon from the top of a mountain.
(The Hebrew word translated as "circle" in Isaiah 40:22 is chuwg, which means "circle" not "sphere." Strong's Concordance: "circle"..."describe a circle." Hebrew and Aramaic Lexicon of the Old Testament: "Circle...the earth conceived as a disc, Is 40:22." Hebrew-Aramaic and English Lexicon of the Old Testament: "draw round, make a circle." If Isaiah actually meant "sphere" he could have used the Hebrew word duwr, meaning "ball" as he did in Isaiah 22:18.)
(But what is particularly significant is that the Jewish scholars who translated the Hebrew scriptures into the Greek Septuagint during the third century B.C. used the Greek word gyron--γῦρον--meaning circle, instead of "sphaíra"--σφαίρα--meaning sphere, for their translation of the Hebrew chuwg in Isaiah 40:22. So those scholars CLEARLY understood what Isaiah was saying, in contrast to those today who ignorantly say that chuwg means "sphere.")
Underneath the flat disk of the earth is the abyss, the bottomless pit, which is referred to several times in the Bible (ex. Rev. 9:1,2).
That is also what is being referred to in Job 26:7 when it says that the earth hangs over nothing. (The Hebrew word translated "upon" in the KJV also means "over.") The actual sphere of the earth in space is not "suspended' or "hanging" "over" or "upon" nothing. It is orbiting the sun at 66,700 miles per hour. If the earth can be considered "hanging" over anything, it is the sun, which certainly is NOT nothing.
Several other verses in the Bible also indicate the earth is flat, such as Nebuchadnezzar's vision in Dan 4:10-11 (the tree could not be seen from all the earth if it were not flat). Dan 2:28 states that the visions of Nebuchadnezzar are from God. If the biblical god says the biblical earth is flat, it must be flat.
The original Hebrew word translated as firmament is raqiya, which is a noun derived from the Hebrew word raqa. That word is a verb meaning "to beat out," and is used in the bible in reference to beating out metal into plates or expanses of the metal (as in Exodus 39:3). So raqiya, as a noun, would literally mean "that which is beaten out."
The biblical firmament, or sky, is therefore a solid, beaten out expanse or vault set on the rim of the flat disk of the earth. That is confirmed in Job 37:18, which states:
"Can you beat out the vault of the skies as he does,
hard as a mirror of cast metal?" (New English Bible).
There, the Hebrew word translated as "beat out" (or "spread out" in other versions) is, as noted above, raqa.
The solid, or firm, nature of the biblical firmament is also indicated by Proverbs 8:27-28:
27. When he established the heavens...
28. When he made firm the skies above... [ASV, ESV, NRSV, NASV, NAB]
The Hebrew word translated as "firm" there is amats, which has a meaning of "be hard".
The solid vault of heaven is also implied in verses such as Deut 4:32:
"Ask now about the former days, long before your time, from the day God created man on the earth; ask from one end of the heavens to the other. Has anything so great as this ever happened, or has anything like it ever been heard of?"
The "ends of heaven" would be the base of the vault of heaven where it rests on the rim of the disk of the earth.
See also Ps 19:1-6, Isaiah 13:4-5, and, particularly, Matt 24:31, which states Jesus as saying:
"And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other."
The elect are those on the earth who will be saved in the last days. Jesus is saying there that the angels would gather the elect from all over the earth, from one side, or end, of the base of the vault of heaven to the other side or end, from the east, west, north, and south, which is what the reference to the four winds means.
Isaiah 34:4 likewise indicates that the firmament is solid rather than being the upper atmosphere or the emptiness of space.
"the skies will roll up like a scroll,
and all the stars will fall"
The material making up the vault of the firmament will roll up, and the stars that were attached to it will fall to the earth.
The solid vault of heaven has a specific purpose in the Bible. As the Genesis story in chapter one indicates, the whole cosmos consisted of water before the creation process began. As it is described in the Genesis creation story, god created the firmament of heaven on the second day so it would hold back the waters above the firmament when he created the earth on the third day.
6 And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so.
So that passage clearly indicates that the purpose of the solid firmament was to hold back the waters above the heaven. Furthermore, those waters continued to exist in the biblical cosmos, as indicated by Psalm 104:1-3 and Psalm 148:4. Also, the firmament of heaven has windows that are opened to let the water above the firmament fall as rain, as indicated by Genesis 7:11 and 8:2,
The stars in the biblical cosmos are just lights set in the firmament. As mere lights in the sky, they will fall to the earth in the Last Days (Matt 24:29), which conflicts with finding that the actual stars are other suns and many times larger than the earth.
So, according to the Bible the earth is a flat, immovable disk, supported by pillars and covered with a solid vault of heaven, the rim of which is is resting on the perimeter of the disk of the earth, and the stars are just lights set in the vault of heaven.
That this is the correct view of the biblical cosmos is shown by the fact that it describes a structure with parts that are fully consistent with each other. That structural consistency indicates that it accurately represents the cosmos as conceived by the ancient Hebrews and as its writers incorporated that view in the Bible.
In addition, according to the Bible, earth is the centerpiece of creation and in the Last Days god will destroy the earth and the heavens and create a new heaven and earth as part of his plan for mankind. (2 Peter 3:10-13).
Is that really realistic considering the vastness of the universe and its enormous number of galaxies? In the actual universe that science has uncovered, the earth is an insignificant mote, even more insignificant than the size of an atom is with respect to the size of the earth. Watch the three-minute video below.
https://www.youtube.com/watch?v=_9gy_BOnEOM&app=desktop
It is the height of geocentric IDIOCY to think that the whole universe was created merely for the sake of the earth and its inhabitants.
All of which goes to show that the cosmos that the god of the Bible is described as having created does not exist, and since that non-existent cosmos is an attribute and manifestation of that god, that god does not exist either.
So, we atheists need no alternative hypothesis on how this universe exists, because there is no alternative explanation since the alternative explanation that you have provided has nothing to do with reality.