sight is necessary to understand the cosmos
If with sight you mean "capability of elaborating electromagnetic waves in the range of the visible spectrum", the statement is simply wrong. We have just got the report that the first image of a black hole event horizon was taken thanks to observation in the radio-frequencies.
So, no, sight is not strictly necessary to understand the cosmos. It is true that the broader spectrum one can analyze the more information can harvest, but lacking a fraction of the spectrum is no showstopper.
Also on a human scale, several space missions have succeeded in exploring space without having a camera for exploration. Just think of the Sputnik: it didn't have a camera, so technically it was blind.
And even we don't need to view something to understand it. Whoever takes calculus at a university level can describe your with extreme precision the properties of a multidimensional surface without visualizing it, just by studying the function representing it. And, if you object that calculus is not exactly a standard knowledge, even visually impaired people get a good understanding of the world without seeing it.