What were Yisrael shown at Har Sinai that made them know ... ?
Rashi (citing the Pesikta): When Hashem gave Yisrael the Torah, He opened the seven Heavens and they saw that He was alone (that there were no other beings there beside Him). 1
Seforno: They were shown all this to impress upon them that Hashem is Elokim - He preceded the world (and that He therefore created it). 2
Targum Yonasan: It refers to the signs and miracles mentioned in the previous Pasuk. 3