Space between ImageButton Android

I wanted to leave a space between the ImageButton, and a space between the ImageButton and the sides, but I can not. I would like the application to look something like this:

ButsheisthiswaywiththeImageButtonalltogether,andpastedonthesideofthescreenofthecellphone

<?xmlversion="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"


tools:context="com.example.tulio.exercicio3.MainActivity">

<TextView
    android:id="@+id/textView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="RGM:XXXX"
    android:gravity="center"
    android:textColor="@android:color/darker_gray"
    android:textSize="24sp"
    android:typeface="normal"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent" />


<TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Fulann"
    android:gravity="center"
    android:textColor="@android:color/darker_gray"
    android:textSize="24sp"
    android:typeface="normal"
    android:id="@+id/txt"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    android:layout_below="@+id/textView"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:layout_marginRight="12dp"
    android:layout_marginEnd="12dp" />


<Button
    android:id="@+id/button"
    android:layout_width="100dp"
    android:layout_height="95dp"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"

    android:layout_below="@+id/txt"
    android:background="@android:color/holo_blue_bright"
    android:text="Principal" />

<ImageButton
    android:id="@+id/button2"
    android:layout_width="100dp"
    android:layout_height="95dp"
    android:layout_below="@+id/txt"
    android:layout_toEndOf="@+id/button"
    android:layout_toRightOf="@+id/button"
    android:height="20dp"
    android:background="@android:color/darker_gray"
    android:gravity="center_vertical"
    android:src="@drawable/auto"
    android:text="Automoveis" />

<ImageButton
    android:id="@+id/button3"
    android:layout_width="100dp"
    android:layout_height="95dp"
    android:layout_below="@+id/txt"
    android:layout_toRightOf="@+id/button2"
    android:background="@android:color/darker_gray"
    android:gravity="center_vertical"
    android:src="@drawable/portateis"
    android:text="Portáties" />

<ImageButton
    android:id="@+id/button4"
    android:layout_width="100dp"
    android:layout_height="95dp"
    android:background="@android:color/darker_gray"
    android:src="@drawable/empresarial"
    android:text="Empresarial"

    android:layout_below="@+id/button"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<ImageButton
    android:id="@+id/button5"
    android:layout_width="100dp"
    android:layout_height="95dp"
    android:text="Residencial"
    android:background="@android:color/darker_gray"
    android:src="@drawable/residencial"
    android:layout_below="@+id/button2"
    android:layout_toRightOf="@+id/button"
    android:layout_toEndOf="@+id/button" />

<ImageButton
    android:id="@+id/button6"
    android:layout_width="100dp"
    android:layout_height="95dp"
    android:text="Viagem"
    android:background="@android:color/darker_gray"
    android:src="@drawable/travel"
    android:layout_below="@+id/button3"
    android:layout_toRightOf="@+id/button2"
    android:layout_toEndOf="@+id/button2" />


    </RelativeLayout>