Add the graham cracker crumbs to a bowl along with the melted butter. Stir to combine and press the crumb mixture into the bottom of an 8-inch tart pan and up the sides of the pan. Place in the refrigerator until ready to use.

In a large bowl, add in the sweetened condensed milk, half & half, and the juice and zest of 3 lemons. Using a hand mixer or stand mixer, beat the mixture on medium-high speed for about 3 more minutes or until the filling becomes slightly thick and glossy.

Pour the filling into the prepared crust and refrigerate for about 6 hours or overnight.

Once the tart has thickened, spread the whipped topping on top. To make the galaxy pattern, add 3 drops of blue food coloring and 3 drops of purple food coloring on the whipped topping and swirl the colors together with a knife. Continue until the entire tart is covered with this pattern. Add a few silver sprinkles and garnish with lemon slices.

Slice, serve, and enjoy!