Informasjonen om det grå stoffet er tilgjengelig.

Because Norwegian often adds the definite article to the end of the noun.

• informasjon = information
• informasjonen = the information

So Informasjonen means the information, not just information.

This is very common in Norwegian:

• bok = book
• boka / boken = the book

Because grå describes the noun, so Norwegian uses the full definite noun phrase:

• stoff = matter / substance / material
• stoffet = the matter
• det grå stoffet = the grey matter

When a noun is definite and has an adjective before it, Norwegian usually uses double definiteness:

• a separate definite article before the adjective: det
• the definite ending on the noun: -et

So:

• et grått stoff = a grey matter/substance
• det grå stoffet = the grey matter/substance

Good question. This is because adjectives change form depending on whether the noun phrase is indefinite or definite.

With a neuter singular indefinite noun:

• et grått stoff = a grey substance

But with a definite noun phrase:

• det grå stoffet = the grey substance

In definite phrases, adjectives usually take the so-called weak form, which is often just the basic dictionary form. So grått becomes grå here.

Compare:

• et stort hus = a big house
• det store huset = the big house

Because stoff is a neuter noun.

Its forms are:

• et stoff = a substance / a material
• stoffet = the substance / the material

When a neuter noun is definite and has an adjective, the phrase uses det:

• det grå stoffet

Compare with other genders:

• den røde bilen = the red car
• det røde huset = the red house
• de røde bilene = the red cars

So det matches the gender/number pattern of the noun phrase.

Here om means about.

So:

• informasjonen om det grå stoffet = the information about the grey matter

This is a very common use of om:

• en bok om språk = a book about language
• en artikkel om historie = an article about history

Be careful: om can also have other meanings in other contexts, such as if or in/about (time), but here it clearly means about.

Because the subject is singular.

• Informasjonen is singular
• so the predicate adjective is singular too: tilgjengelig

Compare:

• Informasjonen er tilgjengelig. = The information is available.
• Opplysningene er tilgjengelige. = The pieces of information are available.

So tilgjengelig is the singular form, and tilgjengelige is the plural/definite form in many contexts.

It is an adjective, meaning available or accessible.

Here it comes after er, so it works as a predicate adjective:

• er tilgjengelig = is available

You can also use it before a noun:

• tilgjengelig informasjon = available information

Typical forms:

• tilgjengelig = singular common gender / predicate base form
• tilgjengelig(e) = plural or some definite contexts
• tilgjengeligt is generally not used; adjectives ending in -ig usually do not add -t in the neuter the way many other adjectives do

So:

• stoffet er tilgjengelig
• informasjonen er tilgjengelig
• ressursene er tilgjengelige

Yes. Stoff is a flexible word in Norwegian.

It can mean:

• material / fabric
• substance
• matter
• sometimes even content or subject matter, depending on context

So the exact meaning depends heavily on context. In this sentence, with det grå stoffet, it refers to grey matter, not cloth.

That is very normal in Norwegian: one noun can cover several related meanings.

No, this is normal main-clause word order.

Structure:

• Informasjonen om det grå stoffet = subject
• er = verb
• tilgjengelig = complement

So it follows the usual pattern:

Subject + verb + complement

That is the standard word order in an ordinary statement:

• Boken er interessant.
• Huset er stort.
• Informasjonen om det grå stoffet er tilgjengelig.

If you wanted an indefinite version, you would remove the definiteness from the nouns.

For example:

• Informasjon om grått stoff er tilgjengelig.

That would mean something like:

• Information about grey matter is available

Notice the changes:

• Informasjonen → Informasjon
• det grå stoffet → grått stoff

This shows the difference between:

• definite: det grå stoffet
• indefinite: grått stoff

The letter å is a separate letter in Norwegian, and it is not pronounced like English a.

In grå, å sounds roughly like the vowel in British English law or saw, though not exactly the same in every dialect.

So grå is approximately:

• groh / graw (very roughly)

A few useful points:

• å is always a vowel
• it is treated as its own letter in the alphabet
• grå means grey

So det grå stoffet contains a very common Norwegian color adjective.