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The simplest way to look at this is that energy can also reveal itself in negative forms. Don't think of it as something only positive, but also there's a negative part of it in the universe that's not directly visible.
For example, we have good reasons to believe that the total energy in the universe adds up to zero. We also have experiments that show that energy is not conserved in very small scales. For example, beta decays produce particles (W-boson) that are about 80 times heavier than the constituents for a short time.
This is OK because of the Heisenberg uncertainty principle, which trades energy for short time periods. In addition to this, we also have good models for quantum chromodynamics (quark physics) that shows that vacuum energy fluctuates and is not constant. Also above that all, we know that gravity only attracts!! Why is that the case? Some scientists see this as only negative energy, unlike electromagnetic and other forces.
So you see, the conservation of energy is not that strict + we have lots of evidence showing that a zero energy state is not a stable state at all. From that point, we cannot really say it's impossible that energy is born by itself in the universe, but only say it's probable.
Note: This is a fairly complicated topic and you need to do physics for very long years to learn all this stuff. So be ready to do that, even if you're not a physicist. I'm avoiding going into details like why energy would not remain around zero. Short answer: CP violation. The universe is not 100% symmetric in time.