Learn Russian: Мой брат хочет знать всё о космосе. - My brother wants to know everything about space.

всё means "everything" and is the neuter singular form of the pronoun весь. Here it acts as the direct object of the verb знать (to know). When this pronoun stands alone to mean "everything" in a general sense, Russian always uses the neuter form. Its Accusative form is exactly the same as its Nominative form.

What happens if I confuse всё and все?

It changes the meaning entirely! всё (pronounced "vsyo") is neuter singular and means "everything." все (pronounced "vsye") is plural and means "everyone" or "all." Even though Russians often skip writing the dots over the ё in everyday text, you must remember the pronunciation difference so you do not accidentally say your brother wants to know "everyone" about space.

Why is the verb form хочет?

It comes from the verb хотеть (to want), which has a tricky stem change. In the singular forms (I, you, he/she/it), the stem changes to хоч- and uses first-conjugation endings (like он хочет). However, in the plural (we, you, they), it keeps the хот- stem and uses second-conjugation endings (like они хотят).

Why does космос end in -е here?