I remember watching an anime when I was a child, with vague memories ... taking the protagonist and his very good brother to outer space and meeting aliens who invaded the earth.

This one?

192, the mystery of the universe invaded the earth. Humans who have no resistance to it have fallen into endless panic after witnessing its powerful destructive power.

Shortly after the Knight of the Universe (8 photos) incident, the outer space development base unexpectedly rescued a strange human falling from the sky, and he was able to turn into a warrior in hard armor to compete with the monster. Strangely, humans claim to have lost their memory because of the impact, but they know the name of mysterious life-ladame. What is even more puzzling is that his transformed appearance is strikingly similar to that of the soldiers who led the invasion of Ladame monster. However, in any case, dying human beings still grasped this life-saving straw-perhaps an angel sent by God, and stayed in the outer space development base as D-boy.