Blessed be the Name of His glorious Majesty forever and ever.
This familiar verse doesn’t appear in the Torah, so it actually isn’t a verse at all! And yet we recite it in between the verses of Shema (Deut. 6:4) and v’ahavta (Deut. 6:5 – so perhaps we should consider it Deut. 6:4 ½!). The midrash in Devarim Rabbah 2:25 tells us that when Moshe visited Heaven to receive the Torah, he heard the angels singing this praise. Upon his return, he taught it to the Jews but the practice became to say it quietly. The metaphor for this is a man who stole jewels from the king’s palace, who then gave them to his wife with instructions not to wear them in public. The exception to this rule is Yom Kippur, when the Jews themselves are considered like angels. (The phrase’s Aramaic analog, Yehei shmei rabbah…, is recited loudly because the angels don’t understand Aramaic, as per Talmud Shabbos 12b.)