El universo parece enorme.

In Spanish, you normally use the definite article with a noun like this when talking about it in a general, specific sense: el universo = the universe.

So:

• El universo parece enorme. = The universe seems enormous.

Leaving out el here would sound unnatural in standard Spanish.

Universo is a masculine noun, so it takes the masculine singular article el.

A good clue is the ending -o, which is often masculine in Spanish:

• el libro
• el cielo
• el universo

Like many nouns, gender is a grammatical category, so it is something you learn together with the word: el universo.

Parece is the 3rd person singular present form of parecer, which usually means to seem or to appear.

So here:

• El universo parece enorme.
• Literally: The universe seems enormous.

It does not necessarily mean only visual appearance, like English looks sometimes does. It can express an impression or judgement.

Because the subject is singular: el universo.

The verb must agree with the subject:

• el universo parece = singular
• los universos parecen = plural

So parece matches el universo.

Because Spanish does not need a subject pronoun here. The subject is already given explicitly as el universo.

In English, you might think in terms of:

• The universe seems enormous.

There is no need to add él, and in fact Él universo would be wrong, because él means he and is a pronoun, while el here is the article the.

Because enorme is acting as a predicate adjective after the verb parecer.

This is similar to how adjectives work after ser or estar:

• Es grande.
• Está cansado.
• Parece enorme.

So the structure is:

• subject + parecer + adjective

Here, enorme describes how the universe seems.

Some Spanish adjectives have the same singular form for masculine and feminine. Enorme is one of them.

So you get:

• el universo enorme / parece enorme
• la casa enorme / parece enorme

The adjective only changes in the plural:

• enorme → singular
• enormes → plural

Examples:

• El universo parece enorme.
• Las galaxias parecen enormes.

This is an important nuance.

• El universo es enorme = The universe is enormous.
  This states it as a fact.

• El universo parece enorme = The universe seems enormous.
  This presents it as an impression or perception.

So parecer adds a sense of appearance, impression, or viewpoint, rather than a direct statement.

Yes, but the meaning changes slightly.

• parece enorme = seems enormous
• se ve enorme = looks enormous

Se ve enorme is often more visual, as if you are describing how something appears to the eye.

Parece enorme is broader and can refer to a general impression, not only visual appearance.

In standard Spain Spanish, parece is pronounced approximately:

• pa-RE-the

More roughly:

• pa as in pah
• re with a tapped Spanish r
• ce with the th sound of English think

That final c before e is pronounced th in most of Spain.

So:

• parece → approximately pah-REH-theh

In many Latin American varieties, it would sound more like pa-RE-se.

Because enorme is an adjective, not a noun.

After parecer, Spanish uses an adjective directly:

• parece enorme
• parece interesante
• parece imposible

You would not say parece un enorme here, because that would be ungrammatical.

Yes. Parecer can be used in several common patterns, such as:

• parecer + adjective
  El universo parece enorme.

• parecer + noun
  Parece un sueño. = It seems like a dream.

• parecer + infinitive
  Parece crecer. = It seems to grow.

• parecer que + clause
  Parece que va a llover. = It seems that it’s going to rain.

In your sentence, it is the parecer + adjective pattern.

Yes. Usually:

• grande = big / large
• enorme = enormous / huge

So enorme is more emphatic than grande.

Compare:

• El universo parece grande. = The universe seems big.
• El universo parece enorme. = The universe seems enormous.

The second one sounds stronger.