There was light without a sun throughout eternity past, for God, though omnipresent, has always manifested himself in his throne room in unapproachable light.
There was light without a sun called forth by God to shine on the watery dark world after it was created. Only later would the sun be given to govern the light.
God once gave the world a vivid display of light without sun in the dwellings of the children of Israel on a dark night in Egypt for all who had spread the blood of a spotless lamb on their doorposts as the death angel passed by.
When Peter and John were taken up on the high mountain at what we call “the transfiguration,” Christ’s face “shone like the sun, and his clothes became white as light . . . He was still speaking when, behold, a bright cloud overshadowed them,” for they were in the presence of glory. Our future is all about such glory.
And, there is light in the eternal city now, the New Jerusalem, which will come down at the end of time. No sun will be needed there.
“And the city has no need of sun or moon to shine on it, for the glory of God gives it light, and its lamp is the Lamb. By its light will the nations walk, and the kings of the earth will bring their glory into it, and its gates will never be shut by day—and there will be no night there.” Revelation 21:23-25 ESV
You may doubt such things have happened or will happen. If you doubt, doubt them all, for if any one is possible, they are all possible. And if you, like me, believe in God with whom nothing is impossible, you will rest and rejoice in their reality.