What is the universe, according to Christianity?
There is no reason to suppose that any Christian denomination would not accept the classic definition of what the universe is at a scientific level: ”the totality of existence".
The physical universe is defined as all of space and time (collectively referred to as spacetime) and their contents. Such contents comprise all of energy in its hvarious forms, including electromagnetic radiation and matter, and therefore planets, moons, stars, galaxies, and the contents of intergalactic space. The universe also includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, and relativity.
The universe is often defined as "the totality of existence", or everything that exists, everything that has existed, and everything that will exist. In fact, some philosophers and scientists support the inclusion of ideas and abstract concepts—such as mathematics and logic—in the definition of the universe. The word universe may also refer to concepts such as the cosmos, the world, and nature. - Universe
Most Christians would be at ease with the above definition as to what the Universe actually is. How it came into matter may be a different subject matter altogether. What is sure is that at one point of time God created the universe and all that it contains.
The classic Christian doctrine of creation is expressed as creation ex nihilo (out of nothing) is widely accepted. The writer who most thoroughly developed this concept was Augustine. He said God spoke the universe into being out of nothing. God did not take eternally preexisting matter or substance and reshape or reconfigure it into the present world. His creative activity is not like that of human designer or worker.
In the beginning God created the heaven and the earth. - Genesis 1:1
The Bible really does not give us a definition of the universe, but Galileo Galilei makes a simple yet profound statement about the universe:
The Bible shows the way to go to heaven, not the way the heavens go. - Galileo and the Interpretation of the Bible
What Happened Before God Created the World?
How He created the heavens and the earth may remain a mystery even after we are in heaven. What He was doing before He created everything is another completely unanswerable question.
There was no time before creation.
Saint Augustine had two answers to those who asked what was God doing before creation. Jokingly he said, God was preparing Hell for people who ask such questions. On a serious level, he noted there was no time before God created and hence the question is meaningless. When God created the heavens and the earth He also created space and time. Before time began there was only eternity. God is a timeless being and time only began with His creation of the universe.