If I understand your question correctly, why do we (that is, people) exist, or why are there living things on at least one planet in our universe (Earth), those are ambitious questions that have scientists, philosophers, and religious leaders awake at night as long as those kinds of thinkers have been around.

Speaking as a scientist, we have very good explanations for how life arose on our planet roughly 3.5 billion years ago, and how species have changed and multiplied into millions of different kinds over the eons.

"Why" questions, particularly about one-of-a-kind ones, such as why are there elephants, or why is Earth bigger than Mars, are often really hard to address using science. Science is a way of coming up with testable explanations about patterns we observe in nature (for example, the paths of planets around the sun are elliptical, what accounts for that?).

There are other ways of explaining what we see or have questions about--some of which are based on belief in various authorities (parents, teachers, celebrities). Many people think that certain sacred texts answer most of our questions. Inconveniently, different cultures/religions frequently accept different answers to the same difficult questions, for example, what happens after we die, or why do we exist? Scientists aren't afraid to say when they don't know the answer to a question. Scientists can chip away at difficult problems for years or centuries, and ultimately make progress, but other things that puzzle us simply aren't suited to scientific methods. Just as tools are only useful for a limited number of tasks, science doesn't work on all questions. For those elusive kinds, we have to turn to other systems of understanding.

I'm impressed that you're grappling with such an enormous question. Don't be frustrated by its complexity. This central mystery has given humans food for thought since our cave-living days.

In science, there is really no theory of why we exist in the universe. There are theories about how we came to exist in the universe, and of course how the universe itself came into existence. What I mean is that the question of why asks about our purpose - what is it that we're here to do? For what or whom do we exist? Modern science does not ask any questions about what our purpose is, just the mechanisms.

In astronomy, that means asking about how solar systems formed, etc.; in biology that means asking about how our body works, e.g. how cells and organs work; in chemistry that means asking about how things react and change each other.

The why of our existence is conventionally left to philosophy and theology, but at some points in history, and depending on the culture, philosophy, theology, and science were not separate fields of study. If you're asking about how, we can look at some theories such as the Big Bang, but if you're asking why, you may want to read more ancient philosophers.

Scientifically, the best theory for why we exist is that the laws of physics allow for us to exist, and the universe is vast enough that anything that can exist inevitably will. As a consequence, we exist because, sooner or later, somewhere, the conditions that give rise to us will happen, and we will exist.

We don't know why the laws of physics are what they are, however.

The theory of evolution is the best theory about why we exist in the universe.

Life on Earth started out as tiny microbes, and this life slowly got more complex, and many new kinds of organisms formed from the tiny microbes, and these different kinds of organisms got more and more complex, and some of the organisms became plants, and other organisms became animals, and the ancient animals slowly changed into many different kinds of animals, and one kind of animal was mammals, and the mammal mothers feed their babies with milk the mothers make in their bodies, and some of the mammals started walking on 2 legs, and these mammals with 2 legs and 2 arms slowly changed into many different kinds of 2-legged mammals, and one of these kinds of 2-legged mammals are our distant ancestors.