In a small saucepan, combine 1 cup of the pomegranate juice with the sugar. Simmer over medium heat until the sugar dissolves and the liquid reduces slightly, which concentrates the flavor and color for that signature Christmas red.
Whisk the agar-agar powder into the remaining half cup of cold pomegranate juice until no lumps remain. Stir this mixture into the warm juice in the saucepan and bring to a gentle boil for two minutes to activate the agar-agar, then remove from heat and let it cool to room temperature.
In a large chilled bowl, beat the heavy whipping cream and the alcohol-free vanilla extract until stiff peaks form. Be careful not to overbeat, you want the cream to be thick and fluffy so the mousse stays light and airy.
Once the pomegranate mixture has cooled but not yet set, gently fold it into the whipped cream using a spatula. Use a slow, circular motion to keep as much air in the mixture as possible, resulting in a beautiful marbled or solid pink hue depending on how much you stir.
Place a silicone semi-sphere mold onto a flat baking sheet. Spoon the mousse into the cavities, smoothing the tops with an offset spatula. Place the tray in the refrigerator for at least 4 hours, or ideally overnight, until the domes are completely firm and set.
Carefully pop the domes out of the silicone molds by pressing gently on the bottom. For an extra luminous look, you can brush them with a little leftover pomegranate reduction or a clear fruit glaze before serving.
