هناك غياب اليوم.

Yes. هناك literally means there in a locational sense, but very often in Modern Standard Arabic it is also used like the English existential there is / there are.

So in هناك غياب اليوم, هناك is not mainly pointing to a place. It is introducing the existence of something:

• هناك غياب اليوم
• There is absence today / There are absences today

This is a very common Arabic way to express existence.

In Arabic, especially in the present tense, sentences often do not use an explicit verb for is / are.

So Arabic can say:

• هناك غياب اليوم

without needing a separate word for is.

This is normal in nominal sentences in the present. If you wanted a past or future meaning, then Arabic would usually use a verb, for example:

• كان هناك غياب اليوم = There was absence today
• سيكون هناك غياب غدًا = There will be absence tomorrow

So the lack of is is completely normal.

غياب means absence or being absent. It is a verbal noun, related to the verb غاب meaning to be absent.

Depending on context, غياب can mean things like:

• absence in general
• absenteeism
• missing attendance
• someone being absent

In a school or workplace context, هناك غياب اليوم might naturally mean:

• There are absences today
• Some people are absent today
• There is absenteeism today

Arabic often uses a singular abstract noun where English may prefer a plural or a more specific phrase.

After existential هناك, Arabic commonly uses an indefinite noun, because the sentence is introducing the existence of something.

So:

• هناك غياب اليوم = There is some absence today / There are absences today

This sounds natural.

If you say هناك الغياب اليوم, it sounds much less natural in this kind of sentence, because الغياب means the absence, which sounds like a specific known absence. Existential there is structures usually introduce something indefinite, just as in English we normally say:

• There is a problem not usually
• There is the problem

So the indefiniteness of غياب is expected.

A careful pronunciation would be:

hunāka ghiyābun al-yawma

A few notes:

• هناك = hunāka
• غياب = ghiyāb
• اليوم = al-yawm

If full case endings are pronounced, then it is often:

• هناك غيابٌ اليومَ
• hunāka ghiyābun al-yawma

In everyday reading or speech, those final case vowels are often not pronounced fully, especially outside very formal contexts.

A fully vowelled version is typically:

هُنَاكَ غِيَابٌ الْيَوْمَ

Why?

• غيابٌ has tanwīn ḍamm because it is indefinite and functions as the main noun being introduced.
• اليومَ is often in the accusative because it functions as an adverbial expression of time, meaning today.

So:

• غيابٌ = an indefinite noun
• اليومَ = adverbial accusative of time

In unvowelled everyday Arabic script, these endings are usually not written.

Here it functions like an adverbial time expression, so the meaning is today.

Although اليوم literally means the day / today, in sentences like this it commonly works as a time adverb:

• اليوم = today
• غدًا = tomorrow
• أمس = yesterday

So in this sentence, اليوم tells you when the absence exists.

Yes, Arabic word order is somewhat flexible, though different orders can change emphasis.

The original:

• هناك غياب اليوم

is a natural neutral way to say There is absence today.

Other possible orders include:

• اليوم هناك غياب = Today, there is absence
  • This puts more emphasis on today.
• غياب اليوم موجود is possible in some contexts, but it is a different structure and less direct here.

For a learner, هناك غياب اليوم is the safest and most natural pattern to remember.

Yes. You could say:

• يوجد غياب اليوم

This also means There is absence today.

Very roughly:

• هناك is extremely common for existential there is/are
• يوجد literally means there exists / there is

Both are correct, though هناك often feels very natural in simple statements like this.

By itself, it does not specify exactly how many people are absent.

Because غياب is an abstract noun, the sentence means something like:

• there is absence today
• there are absences today
• some people are absent today

The exact interpretation depends on context.

If Arabic wants to be more specific, it can say:

• طالب غائب اليوم = A student is absent today
• طلاب غائبون اليوم = Students are absent today
• هناك طالبان غائبان اليوم = There are two students absent today

So هناك غياب اليوم is more general.

Yes, it is a complete and normal Arabic sentence.

Arabic often expresses ideas more compactly than English, especially in present-tense nominal sentences. The sentence has all the parts it needs:

• هناك introduces existence
• غياب tells what exists
• اليوم gives the time

So even though it is short, it is fully meaningful and grammatically complete.