How do scientists believe the universe began?

As far as the existence of universe is concerned, what we have is just lots of theories, some accepted and some not.

Edwin hubble discovered that the universe was expanding, this made us think that there must have been some time when universe didn't exist and later pop into existence. But, theory of relativity shows that time didn't exist before the big bang or just creation of universe...

The basic idea that universe was created by a sudden blast in certain black hole kind substance is just too old and inappropriate as it doesn't include how that ball of matter come into existence.

Now, lets talk about "nothing", nothing is just emptiness. This also implies absence of "free space" because "free space" has many properties like it could ripple and may be "dark energy" too.

Typically, the idea is that "nothing" isn't stable and because of being unstable "free space" pop out of it and virtual particles are formed and destroyed simultaneously. Heisenberg's uncertainty principle implies that energy can be borrowed from a system for a short period of time and so, these virtual particles or matter, anti-matter particle pop in and out of existence within a very short period of time, this has been observed and is called "Casimir Effect.

As per the 2nd law of thermodynamics, the entropy of an isolated system always tends to increase.

If you consider "nothing" it has a higher entropy than something but still, something doesn't become nothing, not until Big crunch at least.

One more look at Heisenberg's uncertainty principle.

what next?

so, the next time someone says "something is better than nothing", just think of it...

Hope it helps!

Cosmic microwave background radiation, the abundance of light elements, and the observed redshift of galaxies are all evidence points toward the Big Bang theory, which holds that the universe started with an event known as the Big Bang some 13.8 billion years ago, marking the beginning of the universe's expansion from a hot, dense state to its current form.