It's not that it's too complex to understand, it's just you're probably imagining it a little incorrectly; which is suppose to happen, because when we get down to the nitty gritty of it, infinity is an absolutely counter-intuitive concept to humans due to instinctual necessity. I'll explain this misconception, and then I'll explain how and why we say the universe is infinite. Firstly, you're thinking of the expansion of space as like two cars being initially parked close together, and driving away from each other. This isn't how the expansion of space works. Imagine it like if you had a parking lot with infinite parking spaces and infinite cars in it. The expansion of space is like if all of these parking spaces grew simultaneously, while the cars remained where they were. The cars themselves are not driving away from each other, but the space between them is increasing. The space is growing more space. Secondly, you're falling into the trap of confusing the universe with the observable universe. The universe is infinite. It always has been and always will be, as per the nature of infinity, and I'll show you how we know this after. This is likely because of Georges Lemaître's (credited with coming up with the Big Bang) famous reference to the early universe as "the primordial atom." This is true, to some extent. By backtracking the universe's expansion, we end up getting to a point from which we cannot measure before, due to quantum limitations on observation. Because of this, we can say that this is the beginning of time, after which the expansion of spacetime carries us to where we are today. However, the expansion of spacetime could not have created energy, thus the energy had to already be existent. Since energy is already existent, and since spacetime expands outwards from everywhere simultaneously, both quantities are infinite, like how in the car example there are infinite cars and infinite parking spaces. The only reason we have an "edge" to the observational universe is because light takes time to travel. Now, to explain how we know the universe is infinite, and what the phrase "the universe is flat" means, I invite you to look at this [url=http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_cmb/12583930-4-eng-GB/Planck_CMB.jpg]picture (same as one embedded above][/url] of the cosmic microwave background, taken by the Planck satellite. Explanation of what the cosmic microwave background is: [spoiler] This is basically the first light we can see in the universe's history. You see, the early universe was obviously very hot, as there wasn't a lot of space for the energy to dissipate. This causes a multitude of differences from the universe today, but the main one to consider is that the energy was too high for individual particles to form, so the forces were unified (this is called grand unification energy, from grand unification theory, or GUT, of you wished to look more in to it). As spacetime expands, the energy density lowers, the forces become distinct from each other and particles form. I'll skip the details, but basically at around 300,000 years after the Big Bang, space had expanded enough to form hydrogen atoms, and because of this, instead of photons simply jumping from free electron to free electron, they were able to decouple from matter and travel anywhere, hence the light we see in this image. [/spoiler] Don't let all the noise in this image fool you, the range of temperature is very very small. If the range were to be just 4 kelvin, the entire image would be the same color. Even with these little fluctuations, imagine it like going up a hill that's 200 meters, and then going down 400 meters, and then going up 200 meters. From point A to point B your altitude hasn't changed because the bumps even out. This image is so uniform, that if Earth were to be this smooth, the tallest mountain would be 100 meters tall. Hence the phrase, "the universe is flat." This causes an issue with the fact that information cannot travel faster than the speed of light. This means that the universe could not have been just a densely packed point, because as it expanded, the density of energy could not possibly remain so constant over these distances due to the constraint on the speed which information can travel. What this implies is that energy must be infinite, and at the time of the Big Bang, it was infinitely dense. If we recall the car analogy, the original idea of a finite energy is like if there were three parking spaces, two of which had cars in them, but as the parking spaces expand, a third car randomly pops into existence it the remaining space, violating the laws of physics. With infinite spaces and infinite cars, the car would have been in that space the entire time, causing no issue when the space expands. You had a great question that touches many topics in astronomy and astrophysics, so I tried to explain it as clearly as possible, which is why this post is so long. If you need any clarification or have any follow up questions, obviously I'm happy to answer.