But, how do you explain such things without first explaining that there IS a universe bigger than what a person can see with the naked eye? Without the technology to prove that stars are stars, or planets are planets, which the telescope did, it's almost impossible to explain that concept.
I'm talking about the fact that it's been advancements in technology that have increased our knowledge, not an inability to grasp the concepts. Until Leevenhoek invented the first crude microscopes, did anyone know that it was tiny organisms unseen by the naked eye that caused disease? How could they when they didn't know they existed? He wasn't even looking for these things when he invented the microscope, he did it for other reasons.
A considerable amount of our current knowledge came as an unexpected result of advancing technology allowing us to observe the previously unobservable, and then changing our view of the physical world.
The progression of knowledge happens in such a way that you can't leap over to concepts like "universe" at least, not in the present sense of the word without an awful lot of supporting concepts coming first. To even get there, you have to have the means to discover new facts and information and then reconceptualize your world.
One man saying, "Let me explain what the sun is to you." without people understanding what inflammable gases were as an underlying concept would be wasting his time, and then you'd have to demonstrate that something invisible exists, such as a gas, and you have to have the technology available to do that.