I do not think there is a limit to what science can explain in the physical or material world. However, I cannot envisage a time when everything will have been explained by science. Early on in my training I realised that each discovery reveals more complex questions. This is what makes science so exciting. Its ability to provide explanations is determined to a large extent by technology and understanding. Explanations based on knowledge available at one time can be debunked or revised when new evidence comes to light.
There are questions that will take a long time for science to fully illuminate – such as the nature of consciousness, even though with smart experiments and functional imaging we are catching glimpses of the inner workings of the brain. I am certain that in my lifetime science will not have a full explanation of how brain activity produces consciousness; however, I think it is a matter of time rather than an impossibility. Most challenging is for us to explain how the universe came to exist from nothing.
The limits I put on what science should try to explain are ethical boundaries, in terms of avoiding harm to people or the environment. There are questions that lie beyond the material world that are best addressed by philosophy as they are not amenable to the scientific method and consequently not worth trying to explain with science, such as: “What is the point of human existence?” Science can explain almost anything, but it is not the answer to everything.