According to the current understanding of the Big Bang theory, the universe originated from an extremely dense and hot state approximately 13.8 billion years ago. It has been expanding and evolving ever since. The conservation laws of physics, such as the conservation of energy and momentum, hold within the framework of the known laws of physics.
In a closed system, no new matter or energy is added from external sources. If we consider the universe as a whole, it can be viewed as a closed system in the sense that it does not receive matter or energy from outside its boundaries.
However, within the universe, various processes can occur that result in the conversion of energy into matter or vice versa. For example, in high-energy environments, such as those present during the early stages of the universe or in the vicinity of black holes, matter and energy can be created or transformed through processes like particle-antiparticle pair production or the conversion of energy into mass.
Moreover, the universe itself is not completely isolated. Although it can be considered a closed system in terms of matter and energy exchange, it is influenced by external factors on a cosmological scale. For instance, the expansion of the universe is affected by the presence of dark energy, a mysterious form of energy that is thought to be driving the accelerated expansion.
It's worth noting that our understanding of the universe and its properties is continually evolving, and there are still many open questions and areas of active research.